  WIMPs with electroweak scale masses (neutralinos, etc.) remain in kinetic
equilibrium with other particle species until temperatures approximately in the
range of 10 MeV to 1 GeV, leading to the formation of dark matter substructure
with masses as small as $10^{-4} M_{\odot}$ to $10^{-12} M_{\odot}$. However,
if dark matter consists of particles with MeV scale masses, as motivated by the
observation of 511 keV emission from the Galactic Bulge, such particles are
naturally expected to remain in kinetic equilibrium with the cosmic neutrino
background until considerably later times. This would lead to a strong
suppression of small scale structure with masses below about $10^7 M_{\odot}$
to $10^4 M_{\odot}$. This cutoff scale has important implications for present
and future searches for faint Local Group satellite galaxies and for the
missing satellites problem.
