document: the first observational evidence that the universe had entered a period of accelerated expansion was obtained when supernovae type ia ( snia ) were found to be fainter than expected @xcite . 
   this fact has been confirmed by many independent observations such as temperature anisotropies of the cosmic microwave background ( cmb ) @xcite , inhomogeneities in the matter distribution @xcite , the integrated sachs  wolfe ( isw ) effect @xcite , baryon acoustic oscillations ( bao ) @xcite , weak lensing ( wl ) @xcite , and gamma - ray bursts @xcite . within the framework of general relativity ( gr ) , 
   the accelerated expansion is driven by a new energy density component with negative pressure , termed dark energy ( de ) . 
   the nature of this unknown matter field has given rise to a great scientific effort in order to understand its properties . 
   the observational evidence is consistent with a cosmological constant @xmath0 being the possible origin of the dark energy ( de ) driving the present epoch of the accelerated expansion of our universe and a dark matter ( dm ) component giving rise to galaxies and their large scale structures distributions @xcite . 
   the dm is assumed to have negligible pressure and temperature and is termed cold . thanks to the agreement with observations the model is commonly known as @xmath0cdm , to indicate the nature of its main components . while favored by the observations , the model is not satisfactory from the theoretical point of view : the value of the cosmological constant is many orders of magnitude smaller than what it was estimated in particle physics @xcite . 
   it was suggested soon that de could be dynamic , evolving with time @xcite . 
   this new cosmological model also suffers from a severe fine - tune problem known as _ coincidence problem _ @xcite that can be expressed with the following simple terms : if the time variation of matter and de are very different why are their current values so similar ? 
   cosmological models where dm and de do not evolve separately but interact with each other were first introduced to justify the currently small value of the cosmological constant @xcite but they were found to be very useful to alleviate the coincidence problem . in this review 
   we will summarize the theoretical developments of this field and the observational evidence that constrains the dm / de interaction and could , eventually , lead to its detection . 
   the emergence of galaxies and large scale structure is driven by the growth of matter density perturbations which themselves are connected to the anisotropies of the cmb @xcite . 
   an interaction between the components of the dark sector will affect the overall evolution of the universe and its expansion history , the growth matter and baryon density perturbations , the pattern of temperature anisotropies of the cmb and the evolution of the gravitational potential at late times would be different than in the concordance model . 
   these observables are directly linked to the underlying theory of gravity @xcite and , consequently , the interaction could be constrained with observations of the background evolution and the emergence of large scale structure . 
   this review is organized as follows : in this introduction we describe the concordance model and we discuss some of its shortcomings that motivates considering interactions within the dark sector . since the nature of de and dm are currently unknown , in sec . 
   [ sec : sec2 ] we introduce two possible and different approaches to describe the de and the dm : fluids and scalar fields . based on general considerations like the holographic principle , we discuss why the interaction within the dark sector is to be expected . in sec . 
   [ sec : sec3 ] we review the influence of the interaction on the background dynamics . 
   we find that a dm / de interaction could solve the coincidence problem and satisfy the second law of thermodynamics . in sec . 
   [ sec : sec4 ] the evolution of matter density perturbations is described for the phenomenological fluid interacting models . in sec . 
   [ sec : sec5 ] we discuss how the interaction modifies the non - linear evolution and the subsequent collapse of density perturbations . in sec . 
   [ sec : sec6 ] we describe the main observables that are used in sec .  [ sec : sec7 ] to constrain the interaction . finally , in sec . 
   [ sec : sec8 ] we describe the present and future observational facilities and their prospects to measure or constrain the interaction . in table 
   [ table : acronyms ] we list the acronyms commonly used in this review . 
   @llll acronym & meaning & acronym & meaning + a - p & alcock - packzynki & ksz & kinematic sunyaev - zeldovich + bao & baryon accoustic oscillations & lbg & lyman break galaxies + cdm & cold dark matter & lhs & left hand side ( of an equation ) + cl & confidence level & lisw & late integrated sachs - wolfe + cmb & cosmic microwave background & lss & large scale structure + de & dark energy & mcmc & monte carlo markov chain + detf & dark energy task force & rhs & right hand side ( of an equation ) + dm & dark matter & rsd & redshift space distortions + eos & equation of state & sl & strong lensing + eisw & early integrated sachs - wolfe & snia & supernova type ia + frw & friedman - robertson - walker & tsz & thermal sunyaev - zeldovich + isw & integrated sachs - wolfe & wl & weak lensing +      the current cosmological model is described by the friedmann - robertson - walker ( frw ) metric , valid for a homogeneous and isotropic universe @xcite @xmath1 , \label{eq : frw - canonical}\ ] ] where @xmath2 is the scale factor at time @xmath3 , the present time is denoted by @xmath4 and the scale factor is normalized to @xmath5 ; @xmath6 is the gaussian curvature of the space - time . 
   we have chosen units @xmath7 but we will reintroduce the speed of light when needed . a commonly used reparametrization is the _ conformal time _ , 
   defined implicitly as @xmath8 . in terms of this coordinate , 
   the line element is @xmath9 . 
   \label{eq : frw - conformal}\ ] ] if we describe the matter content of the universe as a perfect fluid with mean energy density @xmath10 and pressure @xmath11 , friedmann s equations are @xcite @xmath12 where @xmath13 is the hubble function and @xmath14 are the energy density and pressure of the different matter components , related by an equation of state ( eos ) parameter @xmath15 . in terms of the conformal time , the expression @xmath16 is used . 
   usually densities are measured in units of the critical density : @xmath17 with @xmath18 . 
   the curvature term can be brought to the right hand side ( rhs ) by defining @xmath19 . as a matter of convention , a sub - index `` 0 '' denotes the current value of any given quantity . 
   due to the historically uncertain value of the hubble constant , its value is usually quoted as @xmath20kms@xmath21mpc@xmath21 so the parameter @xmath22 encloses the observational uncertainty on the value of the hubble constant .    [ cols="^,^ " , ]      the phenomenological description of the interaction between dark sectors was introduced in sec .  [ 
   sec : sec2 ] and the linear perturbation theory of the model was discussed in sec .  [ 
   sec : sec4 ] . for the sake of simplicity in our subsequent discussion 
   , we will review only models with a de eos parameter @xmath23 . 
   the results for a variable eos have been reported in @xcite . 
   we restrict our analysis to the models that satisfy the stability condition ( sec .  [ 
   sec : sec4.stability ] ) . 
   the interaction kernels were summarized in table  [ table : models ] . 
   the results of the mcmc analysis using different data sets are shown in fig . 
   [ figure2_sec8 ] . when the coupling is proportional to the energy density of the de , the data constraints the value of the interaction parameter to be in the range @xmath24 . 
   when the coupling is proportional to the energy density of the dm or the total energy density of the dark sector , the constraint is much tighter and the coupling are positive at the 68% confidence level ( cl ) . including additional data 
   tightens the constraints on the cosmological parameters compared with the cmb data alone . 
   results for different models can be found in @xcite . 
   the conclusion of these studies shows that interacting dm / de models are compatible with observations . 
   in @xcite it was found evidence of the existence of interaction but with a low cl . 
   more recently the planck collaboration also found that coupled dm / de were in agreement with observations @xcite . in the next section 
   we will further discuss the evidence in favor of an interactions .     as a function of the interaction parameters @xmath25 for models iii and i , ii , respectively . 
   in the left panel we used the cosmological parameters from table v , xi and xii of @xcite . in the right panel , labeled boss , the cosmological parameters are those of table  [ table : boss_parameters ] . 
   the horizontal line corresponds to the boss measured value @xmath26 with the shaded areas representing @xmath27 and @xmath28 cl . ]      together with cmb data , low redshift observations like luminosity distances from snia have been used to test the dm / de interaction . at low redshifts 
   the deviations from @xmath0cdm are not very pronounced and it is easier to establish the existence of a dynamical de component or even an interacting one using high redshift observations . 
   recently , the analysis of boss data presented evidence against the concordance model by measuring the bao in the redshift range @xmath29 from the correlation function of the ly@xmath30 forest from high redshift quasars @xcite . 
   their result indicates a @xmath31 deviation ( from _ _ planck__+wmap ) and @xmath32 deviation ( from act / spt ) from @xmath0cdm at @xmath33 . while the statistical evidence is still not significant , if confirmed , this result can not be explained by a dynamical de component , and it suggests a more exotic type of de . 
   an interacting de appears as a simple and efficient solution to explain the boss result . 
   if de and dm interact and the former transfers energy to the latter , as required to alleviate the coincidence problem ( see sec  [ sec : sec2.direction ] ) and indicated by the data @xcite , it would explain the value of the hubble parameter , @xmath34 , obtained by the boss collaboration , value that is smaller than the expectation from @xmath0cdm @xcite . 
   it would explain the discrepancy of the angular diameter distance at high redshifts . 
   let us now briefly summarize which of the models given in table  [ table : models ] can explain the boss results . 
   let us consider a universe filled only with dm , de and baryons . 
   we can use the hubble parameter obtained from the friedmann equation and compare it with the value obtained by the boss collaboration for different sets of adjusted cosmological parameters . 
   we can also compare the constraints for @xmath35 and @xmath36 given by the boss experiment with constraints from cmb adjusted data using @xmath0cdm and the interaction model . to carry out this analysis we need to establish first the evolution with redshift of the energy densities of each component , specially de and dm since due to the interaction they are not independently conserved . 
   for the models i and ii , they behave as @xcite @xmath37 \rho_d^0}{\xi_2+\omega_d}+\rho_c^0\right\ } . 
   \label{int_de}\end{aligned}\ ] ] the baryonic density is given by the usual expression @xmath38 . 
   for the model iii , the evolution is @xmath39 from these solutions , it is easy to establish that when the energy is transferred from the de to the dm , the energy density of the dm is always smaller than what it would have been in the standard @xmath0cdm model . 
   since @xmath40 is the dominant component at redshifts @xmath41 and it is smaller than in the concordance model , so it would be the hubble parameter , as indicated by the boss data . 
   ccc parameter & bestfit & @xmath42 + @xmath22 & @xmath43 & @xmath44 + @xmath45 & @xmath46 & @xmath47 + @xmath48 & @xmath49 & @xmath50 + @xmath51 & @xmath52 & @xmath53 +    [ table : boss_parameters ]    to make the previous statement more quantitative we took two sets of values for the cosmological parameters @xmath54 and @xmath55 : ( 1 ) the values used by boss collaboration , obtained from the planck collaboration analysis of the @xmath0cdm model and listed in table  [ table : boss_parameters ] , and ( 2 ) the values derived by @xcite by fitting dm / de interacting fluid models to the _ planck _ , bao , 
   snia and @xmath55 indicated above . using both data sets , 
   the hubble parameter at @xmath33 has been computed using eqs . 
   ( [ int_dm],[int_de ] ) for the cosmological models listed in table  [ table : models ] . 
   the results are shown in fig . 
   ( [ fig : all_models ] ) . 
   the right panel corresponds to the cosmological parameters of the boss collaboration and the left panel to those of @xcite . 
   the figure shows the measured value @xmath35 and its @xmath27 and @xmath28 contours . in both panels , 
   the @xmath0cdm model that corresponds to the case of no interaction is always outside the @xmath27cl . while still not significant , 
   it does show that the data prefers an dm / de interacting model with positive interaction . 
   further improvements on the data could help to establish the existence of an interaction . 
   is the critical energy density today . 
   the attractor solutions of @xmath56 does not depend on the initial conditions at the early time of the universe . 
   the purple lines represent the density evolution of cosmological model with different initial conditions . 
   solid circles represent the density contrast @xmath56 today . 
   values change with the initial conditions but they are bounded in two attractor solutions @xmath57 in @xmath58 plane.,width=480,height=432 ]      one motivation to study interacting dm / de models is to alleviate the coincidence problem . only at the present time 
   the dm / de ratio in the @xmath59 model is of order unity , demanding a fine - tuning on the initial conditions at the planck scale of 90 orders of magnitude ( see sec . 
   [ sec : sec1.coincidence ] ) . 
   let us examine if once interacting de models are confronted with observations , the goal can be satisfied . 
   we will concentrate on the analysis of the phenomenological fluid model . 
   the results shown in fig .  [ figure2_sec8 ] suggest that positive coupling parameters are compatible with the data and positive values work in the direction of solving the cosmological coincidence problem @xcite and similar conclusions have also been reached for the field description of the de @xcite . 
   let us particularize our analysis for fluid models , and in particular for model iv . in sec . 
   [ sec : sec2.solution ] we have demonstrated how this model solves the coincidence problem . as illustrated in fig . 
   [ fig : ratios](a ) the ratio has two attractor solutions @xmath60 ; the past solution and future ratios are given in eq . 
   ( [ r+- ] ) . 
   when the coupling is @xmath61 , the ratios behave asymptotically as @xmath62 i.e. , the behavior of the attractor solutions of the ratio @xmath56 only depends on the coupling constant @xmath63 and does not depend on the initial conditions at the planck scale . 
   this solution of the coincidence problem is illustrated in fig . 
   [ figure5_sec7 ] . 
   purple solid lines represent the evolution of the energy densities in units of the critical energy density today , @xmath64 , with different initial conditions . 
   the density contrast @xmath56 at present is different for different initial conditions but all the curves are bounded by the two attractors solutions @xmath65 and @xmath66 . in this particular case 
   we fixed @xmath67 and @xmath68 . during the whole thermal history of the universe , the dm to de ratio takes values within the range @xmath69 ; it changes much less than that in the @xmath59 model , thus the cosmological coincidence problem is greatly alleviated . 
   the discovery that the expansion of the universe is accelerating has led to large observational programs being carried out to understand its origin . 
   new facilities are being designed and built aiming to measure the expansion history and the growth of structure in the universe with increasing precision out to greater redshifts . 
   since the interaction in the dark sectors changes the expansion history of the universe and the evolution of matter and radiation density perturbations , peculiar velocities and gravitational fields , these new facilities will not only test the current period of accelerated expansion but also explain the nature on the interaction between dark sectors . 
   thus , current and future observations could and will be used to set up constraints on interactions between dark sectors and clarify the nature of dm / de interactions . 
   observations of type ia supernova , baryon acoustic oscillations ( bao ) , gravitational lensing , redshift - space distortions and the growth of cosmic structure probe the evolution of the universe at @xmath70 . in parallel 
   , the physics of dm / de interactions at recombination can be probed by the cmb radiation power spectrum while the isw effect and lensing pattern of the cmb sky are sensitive to the growth of matter at lower redshifts . 
   the de task force ( detf ) was established to advise the different u.s . 
   funding agencies on future de research . 
   their report categorized different experimental approaches by introducing a quantitative `` figure of merit '' that is sensitive to the properties of de , including its evolution with time @xcite . using this figure of merit 
   , they evaluated ongoing and future de studies based on observations of baryon acoustic oscillations , galaxy clusters , supernova and weak lensing . 
   the detf categorized the different experiments by their different degree of development . 
   stage i referred to the discovery experiments , stage ii to the on - going experiments at that time when the report was elaborated ( circa 2006 ) , stage iii was defined as the next generation that are currently in full operation . 
   they also looked forward to a stage iv generation of more capable experiments . 
   examples of stage ii surveys are the canada - france - hawaii telescope ( cfht ) legacy survey , with observations of snia @xcite and weak lensing @xcite and that ended in 2009 , the essence @xcite and sdss - ii @xcite supernova surveys and bao measurements from the sdss @xcite . 
   while new observations continue to be expanded and improved with more recent instruments , the chft lensing survey remains the largest weak lensing survey to date .    in this section 
   we will briefly review projects that are currently operating or under construction ( stage iii and iv ) . 
   all of these facilities share the common feature of surveying wide areas to collect large samples of galaxies , clusters , and/or supernovae and they will help clarify the nature of the interaction between dark sectors . 
   more details can be found in @xcite . 
   the existing and planned ground based de experiments collect data on snia , galaxy clustering and gravitational lensing . 
   wide - field imaging is used to measure weak gravitational lensing and clustering of galaxies in bins of photometrically estimated redshifts and wide - field spectroscopy , to map the clustering of galaxies , quasars and the ly-@xmath30 forest and measure distances and expansion rates with bao and the history of structure growth with redshift - space distortions ( rsd ) . 
   type ia supernovae are searched to determine the distance - redshift relation . 
   the 6-degree field galaxy survey ( 6dfgs ) has mapped the nearby universe over @xmath71 deg@xmath72 of the southern sky with galactic latitude @xmath73 . 
   the median redshift of the survey is @xmath74 . 
   it is the largest redshift survey of the nearby universe , reaching out to @xmath75 . 
   the survey data includes images , spectra , photometry , redshifts and a peculiar velocity survey of a subsample of 15,000 galaxies . 
   the final release of redshift data is given in @xcite . 
   the baryon oscillation spectroscopic survey ( boss ) is currently the largest spectroscopic redshift survey worldwide , mapping @xmath76 deg@xmath72 up to @xmath77 . 
   boss is the largest of the four surveys that comprise sdss - iii and has been in operation for 5 years since 2009 . 
   its goals are to measure angular diameter distance and expansion rate using bao , using 1.5 million galaxies @xcite . using ly-@xmath30 lines towards a dense grid of high - redshift quasars , 
   it has pioneered a method to measure bao at redshifts @xmath78 $ ] . 
   the analysis of sdss data release 9 has provided a measurement of the bao scale at @xmath79 with a precision of 2 - 3% @xcite . 
   this survey will be followed by the extended boss ( eboss ) that will be operating for six years and will extend the boss survey to higher redshifts . 
   similar to boss , the hobby - eberly telescope de experiment ( hetdex ) at the austin mcdonald observatory has the goal of providing percent - level constraints on the hubble parameter and angular diameter distance on the redshift range @xmath80 $ ] by using a combination of bao and power spectrum shape information . 
   it will be achieved by surveying 0.8 million @xmath81 emitting galaxies on a field of view of 420deg@xmath72 @xcite . 
   the javalambre physics of the accelerating universe astronomical survey ( j - pas ) is a new astronomical facility dedicated to mapping the observable universe in 56 colors @xcite . 
   the starting date for this multi - purpose astrophysical survey is 2015 . in five years 
   , j - pas will cover @xmath82deg@xmath72 using a system of 54 narrow band and 2 broad - band filters in the range @xmath83 nm . 
   the filter system was optimized to accurately measure photometric redshifts for galaxies up to @xmath84 . 
   the main instruments are a 2.5 m telescope located at el pico del buitre ( teruel , spain ) and a 1.2 giga - pixel camera . 
   the main goals of the survey are to measure angular and radial components of bao from the galaxy clustering , determine the evolution of the cosmic volume from cluster counts and luminosity distances from snia . 
   the filter system will permit to determine the redshifts of the observed supernovae . 
   the camera is not optimized to measure galaxy elipticities so weak lensing studies would require ellipticity measurements obtained from other surveys . 
   the jpas telescope will measure bao from high redshift quasars to achieve a better precision than boss @xcite , open the possibility of using the test described in sec . 
   [ sec : sec7.evidence ] to disproof the concordance model . 
   the panoramic survey telescope and rapid response system ( pan - starrs ) describes a facility with a cosmological survey among its major goals . 
   the final goal is to use four coordinated telescopes to carry out survey of the full sky above dec=@xmath85 @xcite that will go a factor @xmath86 deeper than the sdss imaging survey . 
   the survey will provide data on high redshift sn , galaxy clustering and gravitational lensing . 
   for that purpose , in addition to the wide survey , an ultra - deep field of @xmath87deg@xmath72 will be observed down to magnitude 27 in the @xmath88 band with photometric redshifts to measure the growth galaxy clustering . 
   data from this facility has already been used to constrain the equation of state parameter @xcite . 
   the wigglez de survey is a large - scale redshift survey carried out at the anglo - australian telescope and is now complete . 
   it has measured redshifts for @xmath89 galaxies over 1000deg@xmath72 in the sky . 
   it combines measurements of cosmic distance using bao with measurements of the growth of structure from redshift - space distortions out to redshift @xmath90 @xcite . 
   the atacama cosmology telescope ( act ) operates at 148 , 218 and 277 ghz with full - width at half maximum angular resolutions of @xmath91 @xcite . 
   act observes the sky by scanning the telescope in azimuth at a constant elevation of @xmath92 as the sky moves across the field of view in time , resulting in a stripe - shaped observation area . 
   the collaboration has released two observed areas of @xmath93deg@xmath72 and @xmath94deg@xmath72 @xcite . 
   sky maps , analysis software , data products and model templates are available through nasa legacy archive for microwave background data analysis ( lambda ) .    the south pole telescope ( spt ) is a 10 m telescope designed to map primary and secondary anisotropies in the cmb , currently operating at 95 , 150 , 220 ghz with a resolution with resolution ( 1.7 , 1.2 , 1.0)@xmath95 . 
   the noise levels are 18@xmath96k at 150ghz and @xmath97 larger for the other two channels @xcite . 
   it has observed a region of @xmath98deg@xmath72 . 
   data in the three frequencies were used to produce a radiation power spectra covering the multipole range @xmath99 . at present 
   is the most precise measurement of the radiation power spectrum at @xmath100 at those frequencies ; at those angular scales the signal is dominated by the sz effect and is not so relevant to constrain models of dm / de interaction . 
   a polarization - sensitive receiver have been installed on the spt ; data at 95 and 150 ghz has provided a measurement of the @xmath101-mode polarization power spectrum from an area of @xmath102deg@xmath72 , spanning the range @xmath103 . 
   the resulting power spectra was consistent with predictions for the spectrum arising from the gravitational lensing of @xmath104-mode polarization @xcite . 
   the de survey ( des ) is a wide - field imaging and supernova survey on the blanco 4 m telecope at cerro tololo ( chile ) using the de camera . 
   it has started operations and it will continue for five years . 
   the de spectroscopic survey instrument ( desi ) is a wide field spectroscopic instrument intended to start in 2018 and operate also for five years in the nearly twin mayall telescope at kitt peak ( arizona ) . 
   desi will obtain spectra and redshifts for at least 18 million emission - line galaxies , 4 million luminous red galaxies and 3 million quasi - stellar objects , to probe the effects of de on the expansion history bao and measure the gravitational growth history through rsd . 
   the resulting 3-d galaxy maps at redshift @xmath105 and ly-@xmath30 forest at @xmath106 are expected to provide the distance scale in 35 redshift bins with a one - percent precision @xcite . 
   the imaging survey will detect 300 million galaxies , with approximately 200 million wl shape measurements , almost a two - order of magnitude improvement over the cfhtlens weak lensing survey . 
   approved as a major cosmology survey in sdss - iv ( 2014 - 2020 ) , eboss will capitalize on this premier facility with spectroscopy on a massive sample of galaxies and quasars in the relatively uncharted redshift range that lies between the boss galaxy sample and the boss ly-@xmath30 sample . 
   compared with boss , this new survey will focus on a smaller patch of 7500 deg@xmath72 but it will reach higher magnitudes . 
   it will measure both the distance - redshift relation and the evolution of the hubble parameter using different density tracers ; the clustering of luminous red galaxies ( lgrs ) and emission line galaxies ( elgs ) , quasars and ly-@xmath30 systems to probe the bao scale in the redshift ranges [ 0.6,0.8 ] , [ 1,2.2 ] and [ 2.2,3.5 ] respectively and it will achieve 1 - 2% accuracy in distance measurements from baos between @xmath107 .    the javalambre physics of the accelerating universe astronomical survey ( jpas ) is a new astronomical facility dedicated to mapping the observable universe in 56 colors @xcite . 
   the starting date for this multi - purpose astrophysical survey is 2015 . in five years 
   , jpas will cover @xmath82deg@xmath72 using a system of 54 narrow band and 2 broad - band filters in the range @xmath83 nm . 
   the filter system was optimized to accurately measure photometric redshifts for galaxies up to @xmath84 . 
   the main instruments are a 2.5 m telescope located at el pico del buitre ( teruel , spain ) and a 1.2 giga - pixel camera . 
   the main goals of the survey are to measure angular and radial components of bao from the galaxy clustering , determine the evolution of the cosmic volume from cluster counts and luminosity distances from snia . 
   the filter system will permit to determine the redshifts of the observed supernovae . 
   the camera is not optimized to measure galaxy elipticities so weak lensing studies would require ellipticity measurements obtained from other surveys . 
   the jpas telescope will measure bao from high redshift quasars to achieve a better precision than boss @xcite , open the possibility of using the test described in sec . 
   [ sec : sec7.evidence ] to disproof the concordance model . 
   the large synoptic survey telescope ( lsst ) is a wide - field , ground - based telescope , designed to image @xmath108 deg@xmath72 in six optical bands from 320 nm to 1050 nm . 
   the telescope will be located on cerro pachn ( chile ) and it will operate for a decade allowing to detect galaxies to redshifts well beyond unity . 
   its science goals are to measure weak and strong gravitational lensing , bao , snia and the spatial density , distribution , and masses of galaxy clusters as a function of redshift . 
   its first light is expected on 2019 . 
   the square kilometre array ( ska ) is a radio - facility which is scheduled to begin construction in 2018 . 
   the hi galaxy redshift survey can provide us with accurate redshifts ( using the 21 cm line ) of millions of sources over a wide range of redshifts , making it an ideal redshift survey for cosmological studies @xcite . 
   although technically challenging , the ska could measure the expansion rate of the universe in real time by observing the neutral hydrogen ( hi ) signal of galaxies at two different epochs @xcite . 
   wide - field multi - object spectrograph ( wfmos ) is a camera specially devoted to galaxy surveys . 
   it will be mounted atop the 8.2 m subaru telescope on mauna kea ( hawaii ) . 
   one of the science goals of the wfmos camera is high precision measurements of bao . 
   the wfmos de survey comprises two parts : a 2,000 deg@xmath72 survey of two million galaxies at redshifts @xmath109 and a high redshift survey of about half a million lyman break galaxies ( lbgs ) at redshifts @xmath110 that would probe distances and the hubble rate beyond @xmath111 ( see @xcite for more details ) . 
   bingo is a radio telescope designed to detect bao at radio frequencies by measuring the distribution of neutral hydrogen at cosmological distances using a technique called intensity mapping . 
   the telescope will be located in a disused , open cast , gold mine in northern uruguay . 
   it will operate in the range [ 0.96,1.26 ]  ghz to observe the redshifted 21 cm hydrogen line . 
   it will consist of a two - mirror compact range design with a 40 m diameter primary and it will have no moving parts to provide an excellent polarization performance and very low side - lobe levels required for intensity mapping .      currently , the interest on cmb ground experiments is centered on polarization . for a cosmic variance limited experiment polarization alone places stronger constraints on cosmological parameters than cmb temperature @xcite . 
   experiments like sptpol @xcite and quixote @xcite are currently taken data aiming to characterize the polarization of the cmb and of the galactic and extragalactic sources . 
   cmb experiments devoted to measuring polarization from the ground are also being proposed ; the scientific capabilities of a cmb polarization experiment like cmb - s4 have been considered that in combination with low redshift data would be able to constrain , among other parameters , the de equation of state and dark matter annihilation @xcite . 
   satellite surveys usually require a dedicated facility and , consequently , are more expensive than those carried out from the ground . 
   their significant advantage is that , by observing outside the atmosphere , the data usually contains a lower level of systematic errors . 
   the wilkinson microwave anisotropy probe ( wmap ) was a satellite mission devoted to measure cmb temperature fluctuations at frequencies operating between 23 and 94ghz . 
   launched on june 30 , 2001 and operated for 9 years up to the end of september 2010 . 
   the main results and data products of the nine years of operation are described in @xcite . 
   the final 9yr data released was soon followed by those of the planck collaboration . 
   the planck satellite observed the microwave and sub - millimeter sky from august 12th , 2009 to oct 23rd , 2013 in nine frequencies between 30 and 857 ghz , with angular resolution between 33 and 5. its goal was to produce cmb maps both in temperature and polarization . 
   the planck collaboration has released data on cmb temperature anisotropies , thermal sunyaev - zledovich ( tsz ) effect . 
   the measured temperature and polarization , a catalog of sunyaev - zeldovich ( sz ) clusters and likelihood codes to assess cosmological models against the planck data @xcite and other data products can be downloaded from the planck legacy archive @xcite . the temperature - temperature , temperature - e mode and e mode - e mode power spectra are measured up to @xmath112 @xcite , allowing the cmb lensing potential @xcite and the constraint on cosmological models beyond the @xmath0cdm model @xcite . 
   erosita will be a x - ray satellite that will be launched in 2016 . 
   it will perform the first imaging all - sky survey in energy range 0.3 - 10 kev @xcite . 
   the goal of erosita is the detection of @xmath113 galaxy clusters out to redshifts @xmath114 , in order to study the large scale structure in the universe and test and characterize cosmological models including de . in the soft x - ray band ( 0.5 - 2 kev ) , it will be about 20 times more sensitive than the rosat all sky survey , while in the hard band ( 2 - 10 kev ) it will provide the first ever true imaging survey of the sky at those energies . 
   euclid is a european space agency de satellite mission scheduled for launch in 2020 . 
   this mission is designed to perform two surveys : a wide 15,000 deg@xmath72 survey in the optical and near - infrared and a deep survey on 40 deg@xmath72 two magnitudes deeper . 
   these facilities are not independent between each other . 
   euclid will map the extra - galactic sky with the resolution of the hubble space telescope , with optical and near - infrared ( nir ) imaging and nir spectroscopy . 
   photometric redshifts for the galaxies in the wide survey will be provided from ground photometry and from the nir survey . 
   in addition , 50 million spectroscopic redshifts will be obtained . 
   euclid data will allow to measure the expansion history and the growth of structure with great precision . 
   a detailed quantitative forecast of euclid performance has been discussed in @xcite . 
   the data will allow to constrain many different cosmological models ; when the growth factor is parametrized as @xmath115 the value @xmath116 corresponds to the @xmath0cdm model and euclid will measure this parameter with a precision of @xmath117 @xcite . 
   forecasts for other parametrizations of the growth factor and for other magnitudes such as the bias , de sound speed , redshift space distortions are given in @xcite . 
   the wide field infrared survey telescope ( wfirst ) is an american satellite mission that is currently being reviewed and expected to be launch in 2023 . 
   this mission updates and expands earlier proposed missions like the super nova acceleration probe ( snap ) and the joint de mission ( jdem ) . like euclid , 
   one of its primary science goals is to determine the properties of de and in many respects complements euclid . 
   wfirst strategy is to construct a narrow and deep galaxy redshift survey of 2000 deg@xmath72 . 
   both satellites will measure the redshift for a similar number of galaxies and will obtain a comparable precision for the baryon acoustic oscillations derived angular diameter distances and hubble constant redshift evolution @xcite . 
   nevertheless , due to their different observing strategy will allow cross - checks that will help to identify and eliminate systematics . 
   the combination of both data sets will significantly improve the constraints on the dark energy parameters . 
   many synergies will come from cross - correlating data from different observations for instance , euclid , wfirst and ska have similar scientific aims but will carry observations at different wavelengths . 
   euclid and wfirst probe the low redshift universe , through weak lensing and galaxy clustering measurements . 
   the ska has the potential to probe a higher redshift regime and a different range in scales of the matter power spectrum , which are linear scales rather than the quasi - non - linear scales to which euclid and wfirst will be sensitive . 
   the combination of different observations will particularly sensitive to signatures of modified gravity . 
   cross - correlation of different data sets will help to control systematics for the primary science . 
   the ska , wfirst and euclid will be commissioned on similar timescales offering an exciting opportunity to exploit synergies between these facilities . 
   @xcite      the cosmic origins explorer ( core ) is a stage iv full - sky , microwave - band satellite proposed to esa within cosmic vision 2015 - 2025 . 
   core will provide maps of the microwave sky in polarization and temperature in 15 frequency bands , ranging from 45 ghz to 795 ghz , with angular resolutions from @xmath118 at 45 ghz and @xmath119 at 795 ghz , with sensitivities roughly 10 to 30 times better than planck @xcite . 
   the polarized radiation imaging and spectroscopy mission ( prism ) is a large - class mission proposed to esa in may 2013 within the framework of the esa cosmic vision program . 
   its main goal is to survey the cmb sky both intensity and polarization in order to precisely measure the absolute sky brightness and polarization . 
   the mission will detect approximately @xmath120 clusters using the thermal sz effect and a peculiar velocity survey using the kinetic sz effect that comprises our entire hubble volume @xcite . 
   nasa is carrying similar efforts through the primordial polarization program definition team ( pppdt ) that converge towards a satellite dedicated to the study of cmb polarization ( cmbpol ) @xcite .    combing these complementary ground based and space based observations 
   , we would hopefully achieve a better understanding of the nature of dm , de and the interaction within the dark sectors . 
   we thank s. tsujikawa for comments and suggestions . 
   e. a. wishes to thank fapesp and cnpq ( brazil ) for support and a. a. costa , e. ferreira and r. landim for discussions and suggestions . 
   f. a. b. acknowledges financial support from the ministerio de ciencia e innovacin , grant fis2012 - 30926 and the `` programa de profesores visitantes severo ochoa '' of the instituto de astrofsica de canarias . 
   b. w. would like to acknowledge the support by national basic research program of china ( 973 program 2013cb834900 ) and national natural science foundation of china and he wishes to thank j. h. he and x. d. xu for helpful discussions .
summary: models where dark matter and dark energy interact with each other have been proposed to solve the coincidence problem . 
   we review the motivations underlying the need to introduce such interaction , its influence on the background dynamics and how it modifies the evolution of linear perturbations . 
   we test models using the most recent observational data and we find that the interaction is compatible with the current astronomical and cosmological data . finally , we describe the forthcoming data sets from current and future facilities that are being constructed or designed that will allow a clearer understanding of the physics of the dark sector .