MeV dark matter and small scale structure

Weakly interacting massive particles 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^-4M_ to 10^-12M_. 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 10⁷M_ to 10⁴M_. This cutoff scale has important implications for present and future searches for faint local group satellite galaxies and for the missing satellites problem. We also summarize current constraints on the MeV dark matter scenario.