  If dark matter is composed of neutralinos, one of the most exciting prospects
for its detection lies in observations of the gamma-ray radiation created in
pair annihilations between neutralinos, a process that may contribute
significantly to the extragalactic gamma-ray background (EGB) radiation. We
here use the high-resolution Millennium-II simulation of cosmic structure
formation to produce the first full-sky maps of the expected radiation coming
from extragalactic dark matter structures. Our map making procedure takes into
account the total gamma-ray luminosity from all haloes and their subhaloes, and
includes corrections for unresolved components of the emission as well as an
extrapolation to the damping scale limit of neutralinos. Our analysis also
includes a proper normalization of the signal according to a specific
supersymmetric model based on minimal supergravity. The new simulated maps
allow a study of the angular power spectrum of the gamma-ray background from
dark matter annihilation, which has distinctive features associated with the
nature of the annihilation process and may be detectable in forthcoming
observations by the recently launched FERMI satellite. Our results are in broad
agreement with analytic models for the gamma-ray background, but they also
include higher-order correlations not readily accessible in analytic
calculations and, in addition, provide detailed spectral information for each
pixel. In particular, we find that difference maps at different energies can
reveal cosmic large-scale structure at low and intermediate redshifts. If the
intrinsic emission spectrum is characterized by an emission peak, cosmological
tomography with gamma ray annihilation radiation is in principle possible.
