  We reexamine the impact of dark matter (DM) annihilation on the intergalactic
medium, taking into account the clumping of DM particles. We find that energy
injection from the annihilation of the thermal relic DM particles may
significantly raise the gas temperature at high redshifts and leave a strong
imprint on the cosmological 21-cm signal, provided the particle mass is below
~1 TeV. Further, we find that while the energy injection from DM annihilation
could not alone complete the reionization of the Universe, it could make a
significant contribution to the electron optical depth.
