  It has recently been argued that the PAMELA, ATIC and PPB-BETS data showing
an anomalous excess of high-energy cosmic ray positrons and electrons might be
explained by dark matter annihilating in the Galactic halo with a cross section
resonantly enhanced compared to its value in the primeval plasma. We find that
with a very large annihilation cross section the flash of energetic photons and
electron-positron pairs expected from dark-matter annihilation in the first
protohalos that form at redshift z~40 is likely substantial and observable. As
a consequence, bounds on the allowed energy injection into the primordial gas
and the energy density of the diffuse gamma-ray background give rise to limits
on the low-velocity dark matter cross section that can be difficult to
reconcile with this interpretation of the PAMELA, ATIC and PPB-BETS results.
