document: the nature of dark matter remains one of the outstanding questions of modern astrophysics . 
   the success of the cold dark matter cosmological model ( albeit with `` dark energy '' now required : @xmath3cdm ) argues strongly for a major component of the dark matter being in the form of an elementary particle . 
   however , the inventory of baryons which we can observe locally falls far short of the total inferred from observations of the cosmic microwave background fluctuations @xcite , leaving open the possibility that there may be a significant baryonic component of dark matter . 
   furthermore , although @xmath3cdm is very successful in describing the growth of structure in the universe on large scales , we still lack a direct detection of any of the candidate dark matter particles . lacking this decisive piece of observational evidence , some authors have proposed models which include a large component of baryonic dark matter . in particular 
   there have been many papers dealing with the possibility that cold , self - gravitating molecular clouds constitute a major component of the dark matter @xcite . a variety of different forms , including isolated , clustered , and fractal , 
   have been considered for the clouds , but all proposals involve dense gas of high column - density , in contrast to the diffuse gas in the interstellar medium which is easily detected in emission and/or absorption . 
   one of the fundamental predictions of a model featuring dense gas clouds is the gamma - ray emission resulting from cosmic - ray interactions within the clouds @xcite . 
   because of the potentially large total mass of gas involved , this process may yield a diffuse flux in the galactic plane comparable to the flux from known sources for photon energies around 1 gev @xcite . 
   considering the high quality data on diffuse emission acquired by the egret detector aboard the compton gamma ray observatory @xcite , it is worth considering this source of gamma - ray emission in detail as it is possible to use these data to constrain the dark matter models ( see @xcite ; @xcite ) . 
   most previous investigations of this problem have neglected the self - shielding and cascade phenomena which can be important at high column densities @xcite , and have employed emissivities appropriate to the low - density limit . 
   these effects alter the emergent gamma - ray spectrum , and we note that this could be relevant to the observed excess galactic flux above 1 gev @xcite . 
   we have noted elsewhere @xcite that massive ( @xmath4 ) aggregates of dense gas clouds could potentially account for many of the unidentified discrete sources detected by egret @xcite . 
   here we present detailed calculations of the gamma - ray spectra arising from cosmic - ray interactions with dense gas clouds . 
   we have used a monte carlo simulation code , geant4 , developed for simulating interaction events in detectors used in high - energy particle physics . not surprisingly , we find that the predicted spectra differ substantially between high and low column - density clouds , and we discuss the interpretation of our results in the context of the observed galactic gamma - ray emission . 
   our calculations are undertaken for cold , dense molecular gas in clouds of radius @xmath5 cm , similar to those proposed by @xcite to explain the extreme scattering events @xcite during which compact extragalactic radio sources are magnified and demagnified as a plasma `` lens '' moves across the line of sight ( see @xcite for a criticism of this model ) . 
   however , the results of our calculations depend primarily on the column - density of the individual clouds , @xmath6 , under consideration , and their fractional contribution to the galaxy s dark matter halo , and our results can be taken as representative of other models which are characterised by similar values of these quantities . 
   previous calculations of gamma - ray spectra from cosmic - ray irradiation assumed single interactions of protons with the interstellar medium ( @xcite and references therein ) . in order to investigate cosmic - ray interactions with dense gas , where cascade processes and particle transport are important 
   , we have used a monte carlo code , geant4 , to derive gamma - ray production spectra . 
   this code is a general purpose monte carlo code of particle interactions and is widely used for simulation of high - energy particle detectors in accelerator experiments . 
   cross - sections and interactions of various hadronic processes , i.e. , fission , capture , and elastic scattering , as well as inelastic final state production , are parametrized and extrapolated in high and low particle energy limits , respectively . the @xmath7 production in this code , which is important because of the @xmath8 decay that dominates 
   the emissivity of the gas at high energies , has been tested against accelerator data @xcite . 
   initially we experienced one slight difficulty in applying geant4 to our physical circumstance : the low - energy hadron interaction code , called gheisha , did not conserve energy very accurately ( geant4 bug reports no .  171 and 389 ) . 
   a `` patch '' was available for gheisha ( geant ver . 
   4.4.1 ) , but this patch appeared to introduce further problems of its own in the energy deposition distribution ( geant4 bug report no . 
   these difficulties have been overcome by the geant team , and we are not aware of any such problems in the latest release ( geant ver . 
   4.5.1 ) . 
   our calculations assume a spherical cloud of molecular hydrogen of uniform density and temperature ( 10  k ) . the radius of the sphere was assumed to be @xmath9  au . protons and electrons 
   are injected randomly at a surface point of the cloud and particles subsequently emanating from this surface are counted as products . 
   the adopted spectra of cosmic - ray protons and electrons were taken from @xcite ( here we use the `` median '' flux ; note that the units on his equation ( 3 ) should read @xmath10s@xmath11sr@xmath11gev@xmath11 ) , and @xcite , respectively . in figs . 
   [ fig : f1 ] and [ fig : f2 ] we plot these spectra together with observational data . 
   ( note that the adopted spectra are those for cosmic rays in the galaxy , whereas the measured points are subject to the modulating influence of the magnetic field of the solar wind ) . 
   the simulated range of kinetic energy of cosmic rays is from 10 mev to 10 tev . 
   we divided this energy range into four and superposed the resulting spectra with appropriate weight factors in order to increase the simulation statistics at higher energies , considering the rapidly falling spectrum of cosmic rays . 
   the density of molecular hydrogen , @xmath12 , was varied from @xmath13 to @xmath14 g  @xmath15 in factors of 10 . 
   this corresponds to the column density , @xmath16 , of @xmath17 g  @xmath10 , respectively , where @xmath18 is the incident angle of a cosmic ray into a cloud and @xmath19 for random injection . 
   figure [ fig : f3 ] shows the resulting gamma - ray spectra obtained in our geant4 simulation for proton injection into a cloud of column density @xmath20 . 
   ( the quantity plotted is @xmath21 , with @xmath22 being the emissivity as given in  3 . ) 
   the dashed , dotted and dot - dashed lines show the spectral components classified by the parent processes producing gamma - rays , i.e. , @xmath7 decay , bremsstrahlung and positron - electron annihilation , respectively . 
   the latter two components are necessarily omitted in calculations which assume single interactions only ( i.e. the thin material limit ) . 
   the error bars are calculated from monte carlo statistics . 
   although the @xmath7 decay component shows a broad peak at @xmath23  mev ( note that the quantity plotted is the emissivity multiplied by @xmath24 ) and dominates above about 200 mev , the electron bremsstrahlung component broadens the emissivity peak . 
   the bremsstrahlung and annihilation contributions are negligible in the limit of small column - density ; their fractional contribution to the emission is greatest for clouds which are just thick enough ( @xmath25 ) to attenuate the bulk of the incident proton power . 
   this is as expected considering that the interaction mean free path of gev protons is @xmath26  g  @xmath10 and is in accordance with the result of @xcite who treated a similar problem by solving one - dimensional transport equations . 
   the resulting gamma - ray emissivities for clouds of various column densities are shown in figure [ fig : f4 ] ( proton injection ) and figure [ fig : f5 ] ( electron injection ) . here 
   the emissivities are defined for irradiation by cosmic - rays of all species ( see  2.3 , equation ( 3 ) ) ; to take account of the contribution of heavier nuclei than helium , the emissivity due to proton irradiation ( figure 4 ) has been multiplied by a nuclear enhancement factor @xcite of 1.52 @xcite . note that for high densities the monte carlo statistics are rather poor , since the yield itself is low . 
   figure [ fig : f4 ] includes a comparison of our calculated gamma - ray production functions with that of @xcite ( corresponding to the `` thin material '' limit ) . 
   the results are consistent with those of @xcite for column densities less than about @xmath27 g  @xmath10 , except in the energy range @xmath28 mev where the effect of the maximum energy assumed in the monte carlo simulation is evident . 
   we note the very low values of the emissivity at energies @xmath29  mev , for column densities @xmath30 . 
   a slightly steeper spectrum in the @xmath31@xmath32 mev region comes from our omission of the contribution of heavy nuclei , which were taken into account in @xcite 
   . a somewhat surprising feature of these curves is that the power - law index above 1 gev is almost the same as the input cosmic - ray proton flux for column densities less than about 1000 g  @xmath10 ( for higher column densities the statistics of the simulations are not good enough to decide whether this result still holds ) . 
   this is already indicated by @xcite , but is contrary to the expectation of @xcite who suggested a spectral change above 1 gev at the point where self - shielding becomes important .    for some purposes the energy - integrated emissivities are of more interest than their differential counterparts , so we present these quantities in figures [ fig : f6 ] and [ fig : f7 ] , for cosmic ray protons and electrons , respectively . at low column densities , where cascades and self - shielding are unimportant 
   , there is very little variation of the integrated emissivity with cloud column density , and the thin material limit can be adopted for columns less than @xmath33 for protons ( @xmath34 for electrons ) . above this point , however , the emissivity drops rapidly with increasing cloud column - density . in order to gauge the sensitivity of these calculations to the assumed cosmic - ray spectra , 
   we have computed our results for two different incident cosmic - ray proton spectra , and three different incident cosmic - ray electron spectra , as shown in figures 6 and 7 , respectively . 
   the differences are seen to be small in comparison with the variation as a function of cloud column density , but at a fixed column - density the systematic uncertainties associated with the input cosmic - ray spectra are nevertheless significant . 
   @llll@ +   + @xmath6 & @xmath35 & @xmath36&@xmath37 +   + @xmath38 & @xmath39 & @xmath40 & @xmath41 + @xmath42 & @xmath43 & @xmath44 & @xmath45 + @xmath46 & @xmath47 & @xmath48 & @xmath49 + @xmath50 & @xmath51 & @xmath52 & @xmath53 + @xmath54 & @xmath55 & @xmath56 & @xmath57 + @xmath58 & @xmath59 & @xmath60 & @xmath61 + @xmath62 & @xmath63 & @xmath64 & @xmath65 + @xmath66 & @xmath67 & @xmath68 & @xmath69 +    @lllll@ +   + @xmath70&10&100&1000&10000 +   + @xmath6 & @xmath71 & @xmath71&@xmath71 & @xmath71 + @xmath38&@xmath72&@xmath73&@xmath74&@xmath75 + @xmath42&@xmath76&@xmath77&@xmath78&@xmath79 + @xmath46&@xmath80&@xmath81&@xmath82&@xmath83 + @xmath50&@xmath84&@xmath85&@xmath86&@xmath87 + @xmath54&@xmath88&@xmath89&@xmath90&@xmath91 + @xmath58&@xmath92&@xmath93&@xmath94&@xmath95 + @xmath62&@xmath96&@xmath97&@xmath98&@xmath99 + @xmath66&@xmath100&@xmath101&@xmath102&@xmath103 +      using the gamma - ray production spectra obtained in the previous section , we have calculated the diffuse gamma - ray emission from the galaxy as follows . 
   the predicted gamma - ray spectrum for each case is @xmath104 where @xmath105 is the spectrum returned by the simulation in units of photons / mev / primary , for an individual cloud , appropriate to the incident cosmic - ray spectrum . 
   the quantity @xmath106 is the intensity of cosmic rays at a distance @xmath107 along the line of sight , in units of primaries @xmath10  s@xmath11  sr@xmath11 , and @xmath108 is the mean density in gas clouds of column density @xmath6 . 
   the galactic variation of the spectrum @xmath109 is not well constrained by existing data , and consequently we adopt the simplifying assumption that the shape of the cosmic - ray spectra ( both electrons and protons ) is the same everywhere in the galaxy , with variations only in the normalisation . with this assumption 
   it is convenient to recast the calculation as @xmath110 ( @xmath111 ) , where the emissivity is @xmath112 ( @xmath113 ) with @xmath114 the cosmic - ray mean intensity in the solar neighbourhood and @xmath115 is the weighted column density ( @xmath116 ) of the cloud population along the line - of - sight under consideration . 
   this formulation is convenient because the emissivity , @xmath22 , describes the properties of the gas clouds themselves and is independent of the galactic variations in mean dark matter density and cosmic - ray density ; conversely the quantity @xmath117 characterises these properties of the galaxy , and is independent of the properties of the gas clouds themselves . 
   the emissivity shown in figures 4 and 5 is the quantity @xmath21 , whereas figures 6 and 7 show @xmath118 . for the inner galactic disk , where we are interested in @xmath119 
   , we need to average over the whole solid angle , @xmath120 , under consideration : @xmath121 . in order to calculate 
   @xmath117 we need to adopt models for both the galactic cosmic - ray distribution and the galactic distribution of the clouds .    the quantity @xmath108 , the density in cold , dense gas clouds , is only weakly constrained by direct observation , because the hypothetical clouds constitute a form of _ dark _ matter . 
   we therefore proceed by adopting a conventional dark matter density distribution for the galaxy , namely a cored isothermal sphere , as our model cloud density distribution , with a fiducial normalisation which is equivalent to the assumption that all of the dark matter is in the form of dense gas clouds . 
   this corresponds to the model @xmath122 in terms of cylindrical coordinates @xmath123 , with @xmath124 . 
   we have adopted a core radius of @xmath125  kpc based on the preferred model of walker ( 1999 ) . 
   ( this choice corresponds to walker s preferred value of cloud column density @xmath126 . ) 
   walker s model exhibits a core radius which is a function of cloud column density , but we have fixed the core radius at 6.2  kpc for all of our computations . 
   this choice permits more straightforward consideration of the observational constraints because @xmath117 is independent of @xmath6 in this case . 
   it then remains to specify the cosmic - ray energy - density as a function of position in the galaxy . 
   @xcite ( hereafter wlg92 ) constructed numerical models of cosmic - ray propagation in the galaxy ; they did not give any analytic forms for their model cosmic - ray distributions , but an appropriate analytic approximation can be deduced from the results which they obtained . 
   they found that the cosmic - ray radial distribution reflects , in large part , the radial dependence of cosmic - ray sources , with a modest smoothing effect introduced by diffusion . 
   we have therefore adopted wlg92 s preferred model ( their model 3 ) for the radial distribution of sources as our model for the radial distribution of cosmic rays .    the various spectra of cosmic - ray isotope ratios considered by wlg92 favour models in which the diffusion boundaries are in the range @xmath127  kpc above and below the plane of the galaxy . 
   we adopt the midpoint of this range . 
   wlg92 do not give a simple functional form for the vertical variation of cosmic - ray density within this zone , so we have simply assumed an exponential model : @xmath128 . we know that in wlg92 s models the cosmic - ray density is fixed at zero at the diffusion boundaries , and consequently the scale height of the exponential should be approximately half the distance to the diffusion boundary , i.e. @xmath129  kpc . 
   these considerations lead us to the model cosmic - ray mean intensity distribution @xmath130 : @xmath131,\ ] ] in terms of cylindrical coordinates @xmath132 . here 
   @xmath133  kpc is the radius of the solar circle , while @xmath134  kpc , @xmath135  kpc and @xmath136 . 
   this distribution has the character of a disk with a central hole .    these models for @xmath108 and 
   @xmath137 allow us to compute the quantity @xmath117 , as per equation 4 , and the resulting variation over the sky is plotted in figure 8 . for reference we give the values of @xmath138 evaluated at the cardinal points , as follows : @xmath139 , @xmath140 , @xmath141 , and @xmath142 . in order to compare with the egret results of @xcite 
   , we have also evaluated the average of @xmath117 over the inner galactic disk : @xmath143 . 
   several physical processes contribute to the observed diffuse gamma - ray intensity . in order of decreasing fractional contribution 
   these are thought to be pion production and bremsstrahlung from cosmic - ray interactions with _ diffuse _ galactic gas , inverse compton emission from cosmic - ray electrons interacting with ambient photons , and an isotropic background , which is presumably extragalactic and due to many faint , discrete sources . 
   the sum of these contributions offers a good model for the diffuse emission which is actually observed in the 100  mev1  gev band @xcite ; however , the @xmath144  gev emission is poorly modeled @xcite with the prediction @xcite amounting to only @xmath145 of the observed intensity . 
   some authors @xcite have used the low - energy ( @xmath146  gev ) data to argue that the agreement between model and data allows no room for any significant unmodeled emission and that these data therefore place tight constraints on any baryonic contribution to the dark matter halo of the galaxy . 
   this line of argument is clearly suspect because of the failure of the same model to account for the high - energy ( @xmath144  gev ) data . 
   however , even if we ignore the high - energy data the calculations presented in  2.3 demonstrate that the gamma - ray constraints on high column density gas clouds in the dark halo are much weaker than those on low column density gas because the emissivity of the latter is much greater ; consequently , we revisit the published constraints in the following section (  3.2 ) , returning to the question of the high - energy data in  3.3 . 
   @xcite argued that the known ( diffuse ) gas accounts for essentially all of the gamma - ray intensity in the @xmath147  mev band observed by the cos b satellite @xcite , and he suggested that any contribution from baryonic clouds should amount to no more than @xmath148 ( @xmath147  mev ) at high galactic latitudes . toward the galactic poles we found (  3 ) @xmath149 , implying that the average emissivity of the material in the dark halo should be @xmath150 for @xmath151 . from table 1 ( or 
   figures 6 and 7 ) we see that this requirement is not met for the models which are in the thin material limit but that any model with @xmath152 is acceptable . in other words 
   the constraint imposed by @xcite permits the galaxy s dark halo to be entirely baryonic , provided the individual components have a column - density @xmath152 . 
   @xcite pointed out that obervations at low galactic latitudes can provide tighter constraints on any baryonic dark halo , because of the higher intensity expected when looking through the cosmic - ray disk edge - on . 
   this point is manifest in the large values of @xmath153 , which we computed in  2.3 ( see also figure 8) . 
   @xcite argued that a suitable constraint on any baryonic component of the dark halo is that it should not contribute more than the uniform background intensity component observed in any given field . 
   the strongest constraint then comes from observations of the ophiuchus region @xcite , for which the requirement imposed by @xcite is an intensity contribution from the dark matter halo of @xmath154 in the band 300  mev @xmath155 500 $ mev . for this line of sight 
   our calculation yields @xmath156 , and in the thin material limit , for which we find @xmath157 ( 300  mev @xmath155 500 $ mev ; see table 1 ) , this corresponds to a predicted intensity of @xmath158 in the 300500  mev band , implying that at most 8% of the dark halo can be resident in low column density clouds . 
   kpc and a spherical halo ) , a difference which is accounted for by their choice of a smaller core radius for the dark halo . 
   @xcite chose a core radius of 3.5  kpc , which yields @xmath159 , whereas we have employed a core radius of 6.2  kpc ( see  2.3 ) . 
   ] however , our calculations extend to clouds of higher column - densities , and we find that for @xmath160 the limit relaxes to the point where all of the dark halo is permitted to be in the form of high column density clouds . 
   the constraints discussed in the previous subsection make reference only to the low - energy ( @xmath146  gev ) gamma - ray data . 
   as noted earlier in this section , the observed intensity of the inner galactic disk at high photon energies ( @xmath161  gev ) is substantially greater than expected @xcite . 
   much effort has been expended on explaining this discrepancy , with most of the attention given to models in which the galactic cosmic - ray electron and/or proton spectra differ from their locally measured values @xcite . however 
   , none of these models offers a satisfactory explanation of the observed mean gamma - ray spectrum of the galactic disk , and consequently a successful match to the low energy data should not be taken to mean that the emission model is correct . in turn 
   this suggests that the constraints formulated by @xcite and @xcite , on the basis of the low - energy data , may be too restrictive . 
   the question then arises as to what constraints the gamma - ray data do in fact place on unmodeled emission , given the current state of understanding of the observed emission . 
   the galactic diffuse emission model used in @xcite is based on @xcite , and contains the following contributions : @xmath162 where @xmath163 is the gamma - ray intensity contributed by cosmic - ray interactions with diffuse atomic hydrogen , and similarly for the ionised and molecular components of the interstellar medium ( hii and @xmath164 , respectively ) . 
   the emissivity for these components @xcite is , of course , computed in the thin material limit , and each component therefore has the same spectral shape , differing only in normalisation . here 
   @xmath165 is the gamma - ray flux by inverse compton emission ( cosmic - ray electrons up scattering low - energy photons ) , and @xmath166 is the isotropic extragalactic background flux . 
   although the atomic and ionised hydrogen components can be observed directly via their line emission , and are thus well constrained , this is not true for the molecular component . 
   the molecular hydrogen column is assumed to be proportional to the co emission line strength , as measured by the columbia co survey @xcite , for example , but the constant of proportionality ( usually denoted by @xmath167 ) is unknown and one is forced to determine its value by fitting to the gamma - ray data . 
   although the uncertainty in the best - fit determination of @xmath168 ( averaged over the whole sky ) is small , the systematic uncertainties are acknowledged to be much larger , `` at least 10%15% '' @xcite . in turn , this estimate of the uncertainty is small in comparison with the differences among the various values of @xmath167 which have been reported in the literature ( see the review by @xcite ) and the likely range of variation in @xmath167 within the galaxy @xcite . 
   although these are substantial uncertainties , molecular hydrogen contributes only a fraction of the total observed gamma - ray intensity roughly 20% of the local surface density of gas in the galactic disk is thought to be in molecular form @xcite so the implied fractional uncertainty in the total galactic emission is perhaps as small as 3% . 
   moreover , uncertainty in @xmath167 affects only the normalisation and not the spectral shape of the predicted emission from diffuse molecular hydrogen , so the observed high - energy excess can not be explained in this way even if @xmath167 could assume an arbitrarily large value . 
   the other main contributions to uncertainty in the diffuse model are associated with ( 1 ) unmodeled spatial variations in the cosmic - ray spectral energy density and ( 2 ) unmodeled spatial variations in the low - energy photon spectral energy density ; the former affects all of the galactic contributions to @xmath169 , whereas the latter affects only the inverse compton component . 
   these uncertainties affect both the normalisation and the spectral shape of the predicted gamma - ray emission ; however , the uncertainties are difficult to quantify .      for our purposes it 
   is not actually necessary to quantify the uncertainties on the model input parameters ; it suffices to use the discrepancy between model and data as a measure of the uncertainty in our understanding of the observed emission . in turn 
   this measure determines the constraints which we can apply to any putative unmodeled emission , such as the contribution from dense gas which we are concerned with here . at photon energies 
   @xmath170 the fractional discrepancy is roughly 60% @xcite , in the sense that the observed emission is 1.6 times larger than the model , and we henceforth adopt @xmath171% of the total observed intensity as our estimate of the unmodeled emission . 
   although this estimate is derived from data at high energies , the effects of the various contributing processes are all very widely spread , and _ the estimate therefore applies independent of photon energy . _ the constraints appropriate to high / low galactic latitudes can now be re - evaluated .    at high galactic latitudes 
   the observed intensity is @xmath172 for @xmath173 @xcite , implying that any unmodeled emission should be @xmath174 in this band . 
   this result is actually slightly stricter than the criterion used by @xcite and thus leads us to tighten our high - latitude constraints , relative to those quoted in  3.2 : the observed high - latitude gamma - ray intensity constrains the amount of low column - density gas to @xmath175% of the total density of the galactic dark halo , with this fraction rising to 100% for gas clouds of column density @xmath176 .    at low galactic latitudes we can make use of the mean intensity of the inner galactic disk , which has been accurately determined by @xcite . 
   for example at 1  gev the mean intensity ( @xmath177 , @xmath178 ) is @xmath179 , and our calculation of @xmath180 for this region yields (  2.3 ) @xmath181 , implying that the emissivity of the galactic dark halo material must be , on average , @xmath182 . by comparison , 
   the actual emissivity of low column - density gas is computed to be (  2.3 , table 2 ) @xmath183 , implying that @xmath184% of the galaxy s dark halo may be comprised of low column density gas . 
   for higher column densities the emissivity falls , and table 2 shows that for @xmath185 the emissivity is only @xmath186 . the gamma - ray data on the inner galactic disk thus indicate all of the galaxy s dark halo to be made of dense clouds of column - density @xmath152 . 
   the constraint we have just given is based on the mean spectrum of the inner galactic disk , in contrast to those given by @xcite who employed limits based on the angular structure of the observed gamma - ray intensity . 
   specifically , @xcite required that the putative contribution of emission from a baryonic component of the galaxy s dark halo be less than that of the isotropic component of the observed intensity ; this procedure seems to us to be less reliable than the procedure we have employed , for two reasons . 
   first , even if the dark halo were spherically symmetric the emission from any baryonic component would not be , both because our point of observation is quite distant from the centre of the galaxy and because the resulting gamma - ray emission is strongly dependent on the galactic cosmic - ray distribution , which in turn is strongly concentrated in the disk of the galaxy . indeed the cosmic - ray distribution appears to correlate with the distribution of interstellar matter @xcite , thus complicating the interpretation of the observed gamma - ray intensity distribution . in particular this coupling leads to gamma - ray emission from a baryonic dark halo being correlated with the diffuse gas column density , even if the dense gas is uncorrelated with the diffuse gas . 
   second , on any given line of sight , such as the ophiuchus field considered by @xcite , a highly structured dark matter halo might exhibit , by chance , a low dark matter column density . 
   the chances of this are good if , as in the case of @xcite , the field is specifically chosen to have a low `` background '' intensity . 
   the gamma - ray spectra arising from cosmic - ray interactions with gas clouds of various column - densities have been calculated using a monte carlo event simulator , geant4 . 
   our calculations reproduce the analytic result in the low column - density limit , where only single particle interactions need to be considered , but exhibit significant differences for clouds of column - density @xmath187 where the emissivity declines substantially for photon energies @xmath188 . 
   the low emissivity of dense gas means that the baryonic content of the galaxy s dark halo is not so tightly constrained by the gamma - ray data as had previously been thought . 
   for @xmath176 we find that the existing gamma - ray data , taken in isolation , do not exclude purely baryonic models for the galactic dark halo . 
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summary: gamma - ray spectra from cosmic - ray proton and electron interactions with dense gas clouds have been calculated using a monte carlo event simulation code , geant4 . 
   such clouds are postulated as a possible form of baryonic dark matter in the universe . 
   the simulation fully tracks the cascade and transport processes which are important in a dense medium , and the resulting gamma - ray spectra are computed as a function of cloud column - density . 
   these calculations are used for predicting the galactic diffuse gamma - ray spectrum which may be contributed by baryonic dark matter ; the results are compared with data from the egret instrument , and used to constrain the fraction of galactic dark matter which may be in the form of dense gas clouds . in agreement with previous authors , 
   we find useful constraints on the fraction of galactic dark matter which may be in the form of low column - density clouds ( @xmath0 ) . 
   however , this fraction rises steeply in the region @xmath1 , and for @xmath2 we find that baryonic dark matter models are virtually unconstrained by the existing gamma - ray data .