  The origin of the cosmic gamma-ray background (CGB) is a longstanding mystery
in high-energy astrophysics. Possible candidates include ordinary astrophysical
objects such as unresolved blazars, as well as more exotic processes such as
dark matter annihilation. While it would be difficult to distinguish them from
the mean intensity data alone, one can use anisotropy data instead. We
investigate the CGB anisotropy both from unresolved blazars and dark matter
annihilation (including contributions from dark matter substructures), and find
that the angular power spectra from these sources are very different. We then
focus on detectability of dark matter annihilation signals using the anisotropy
data, by treating the unresolved blazar component as a known background. We
find that the dark matter signature should be detectable in the angular power
spectrum of the CGB from two-year all-sky observations with the Gamma Ray Large
Area Space Telescope (GLAST), as long as the dark matter annihilation
contributes to a reasonable fraction, e.g., >~ 0.3, of the CGB at around 10
GeV. We conclude that the anisotropy measurement of the CGB with GLAST should
be a powerful tool for revealing the CGB origin, and potentially for the first
detection of dark matter annihilation.
