  We investigate contributions to the extragalactic gamma-ray background (EGB)
due to neutralino dark matter (DM) pair-annihilation into photons, from DM
density enhancements (minispikes) surrounding intermediate-mass black holes
(IMBHs). We focus on two IMBH formation scenarios; our conservative scenario
where IMBHs are remnants of Population-III stars, and our optimistic scenario
here IMBHs are formed in protogalactic disks. In both scenarios, their
formation in pregalactic halos at high redshift lead to the formation of
minispikes that are bright sources of gamma-ray photons. Taking into account
minispike depletion processes, we only sum contributions from a cosmological
distribution of IMBHs with maintained minispikes. Our conservative scenario (BH
mass 10^2 M_sun with a r^{-3/2} minispike) predicts gamma-ray fluxes that are
an order larger than the equivalent flux, using the same DM parameters (mass
100 GeV and annihilation cross-section 3 \times 10^{-26} cm^3 s^{-1}, from the
host halo without IMBH minispikes. Our optimistic scenario (BH mass 10^5 M_sun
with a r^{-7/3} minispike) predicts fluxes that are three orders larger, that
can reach current EGB observations taken by EGRET (DM parameters as above).
This fact may serve interesting consequences for constraining DM parameters and
elucidating the true nature of IMBHs. Additionally, we determine the spectra of
DM annihilation into monochromatic gamma-rays, and show that its flux can be
within observational range of GLAST, providing a potential `smoking-gun'
signature of DM.
