The cosmic X-ray and gamma-ray background from dark matter annihilation
  (Abridged) The extragalactic background light (EBL) observed at multiple
wavelengths is a promising tool to probe the nature of dark matter since it
might contain a significant contribution from gamma-rays produced promptly by
dark matter annihilation. Additionally, the electrons and positrons produced in
the annihilation give energy to the CMB photons to populate the EBL with X-rays
and gamma-rays. We here create full-sky maps of the radiation from both of
these contributions using the high-resolution Millennium-II simulation. We use
upper limits on the contributions of unknown sources to the EBL to constrain
the intrinsic properties of dark matter using a model-independent approach that
can be employed as a template to test different particle physics models
(including those with a Sommerfeld enhancement). These upper limits are based
on observations spanning eight orders of magnitude in energy (from soft X-rays
measured by CHANDRA to gamma-rays measured by Fermi), and on expectations for
the contributions from blazars and star forming galaxies. To exemplify this
approach, we analyze a set of benchmark Sommerfeld-enhanced models that give
the correct dark matter abundance, satisfy CMB constraints, and fit the cosmic
ray spectra measured by PAMELA and Fermi without any contribution from local
subhalos. We find that these models are in conflict with the EBL constraints
unless the contribution of unresolved subhalos is small and the annihilation
signal dominates the EBL. We conclude that provided the collisionless cold dark
matter paradigm is accurate, even for conservative estimates of the
contribution from unresolved subhalos and astrophysical backgrounds, the EBL is
at least as sensitive a probe of these types of scenarios as the CMB. Our
results disfavor an explanation of the positron excess measured by PAMELA based
only on dark matter annihilation in the smooth Galactic halo.
