  We demonstrate that elastic scattering between dark matter (DM) and baryons
can affect the thermal evolution of the intergalactic medium at early epochs
and discuss the observational consequences. We show that, due to the
interaction between DM and baryons, the baryon temperature is cooled after
decoupling from the CMB temperature. We illustrate our findings by calculating
the 21 cm power spectrum in coexistence with a velocity-dependent DM elastic
scattering cross section. For instance, for a DM mass of 10 GeV, the 21 cm
brightness temperature angular power spectrum can be suppressed by a factor 2
within the currently allowed DM-baryon cross section bounded by the CMB and
large-scale structure data. This scale-independent suppression of the angular
power spectrum can be even larger for a smaller DM mass with a common cross
section (for instance, as large as a factor 10 for $m_d\sim 1$ GeV), and such
an effect would be of great interest for probing the nature of DM in view of
forthcoming cosmological surveys.

Cosmology and Nongalactic Astrophysics;High Energy Physics - Phenomenology