  The extragalactic dark matter (DM) annihilation signal depends on the product
of the clumping factor, <\delta^2>, and the velocity-weighted annihilation
cross section, \sigma v. This "clumping factor-\sigma v" degeneracy can be
broken by comparing DM annihilation signals from multiple sources. In
particular, one can constrain the minimum DM halo mass, M_min, which depends on
the mass of the DM particles and the kinetic decoupling temperature, by
comparing observations of individual DM sources to the diffuse DM annihilation
signal. We demonstrate this with careful semi-analytic treatments of the DM
contribution to the diffuse Isotropic Gamma-Ray Background (IGRB), and compare
it with two recent hints of DM from the Galactic Center, namely, ~130 GeV DM
annihilating dominantly in the \chi\chi\ to \gamma\gamma\ channel, and (10-30)
GeV DM annihilating in the \chi\chi\ to b\bar{b} or \chi\chi\ to
\tau^{+}\tau^{-} channels. We show that, even in the most conservative
analysis, the Fermi IGRB measurement already provides interesting sensitivity.
A more detailed analysis of the IGRB, with new Fermi IGRB measurements and
modeling of astrophysical backgrounds, may be able to probe values of M_min up
to 1 M_sun for the 130 GeV candidate and 10^{-6} M_sun for the light DM
candidates. Increasing the substructure content of halos by a reasonable amount
would further improve these constraints.
