  We study the imprint of dark matter (DM) annihilation on the global 21 cm
signal from the Dark Ages to Cosmic Reionization. Motivated by recent
observations, we focus on three DM candidates: (i) a 10 GeV Bino-like
neutralino (ii) a 200 GeV Wino and (iii) a 1 TeV heavier particle annihilating
into leptons. For each DM candidate we assume two values for the thermally
averaged annihilation cross section \sigma v, the standard thermal value \sigma
v_th = 3 x 10^-26 cm^3 s^-1 and the maximum value allowed by WMAP7 data, \sigma
v_max. We include the enhancement of DM annihilations due to collapsed
structures, detailed estimates of energy deposition into the intergalactic
medium (IGM), as well realistic prescriptions for astrophysical sources of UV
and X-ray radiation. In these models, the additional heat input from DM
annihilation suppresses the mean 21cm brightness temperature offset by \delta
Tb ~ few-100 mK. In particular, the very deep \delta Tb ~ -150 mK absorption
feature at ~20 < z < 25 predicted by popular models of the first galaxies is
considerably reduced or totally erased by some of the considered DM candidates.
Such an enhancement in IGM heating could come from either DM annihilations or a
stronger-than-expected astrophysical component (i.e. abundant early X-ray
sources). However, we find that the two signatures are not degenerate, since
the DM heating is dominated by halos several orders of magnitude smaller than
those hosting galaxies, whose fractional abundance evolves more slowly
resulting in a smaller gradient: d \delta Tb/d \nu < 4 mK/MHz in the range \nu
~60-80 MHz. The detection of such signals by future radio telescopes would be
clear evidence of DM energy injection at high-redshifts.
