Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The highest energy particles in the universe: the mystery and its
  possible solutions   The universe should be dark at energies exceeding $\sim 5\times 10^{19}$ eV.
This simple but solid prediction of our best known particle physics is not
confirmed by observations, that seem to suggest a quite different picture.
Numerous events have in fact been detected in this energy region, with spectra
and anisotropy features that defy many conventional and unconventional
explanations. Is there a problem with known physics or is this a result of
astrophysical uncertainties? Here we try to answer these questions, in the
light of present observations, while discussing which information future
observations may provide on this puzzling issue.

10
The categories of this paper are: q-bio
12
The categories of this paper are: nucl-th
32
The categories of this paper are: atom-ph
21
The categories of this paper are: adap-org
1
The categories of this paper are: cs
17
The categories of this paper are: q-fin
23
The categories of this paper are: solv-int
7
The categories of this paper are: hep-th
33
The categories of this paper are: plasm-ph
31
The categories of this paper are: ao-sci
4
The categories of this paper are: cond-mat
27
The categories of this paper are: comp-gas
3
The categories of this paper are: physics
36
The categories of this paper are: cmp-lg
35
The categories of this paper are: mtrl-th
37
The categories of this paper are: supr-con