  Observations of the redshifted 21 cm line from neutral hydrogen will open a
new window on the early Universe. By influencing the thermal and ionization
history of the intergalactic medium (IGM), annihilating dark matter (DM) can
leave a detectable imprint in the 21 cm signal. Building on the publicly
available 21cmFAST code, we compute the 21 cm signal for a 10 GeV WIMP DM
candidate. The most pronounced role of DM annihilations is in heating the IGM
earlier and more uniformly than astrophysical sources of X-rays. This leaves
several unambiguous, qualitative signatures in the redshift evolution of the
large-scale ($k\approx0.1$ Mpc$^{-1}$) 21 cm power amplitude: (i) the local
maximum (peak) associated with IGM heating can be lower than the other maxima;
(ii) the heating peak can occur while the IGM is in emission against the cosmic
microwave background (CMB); (iii) there can be a dramatic drop in power (a
global minimum) corresponding to the epoch when the IGM temperature is
comparable to the CMB temperature. These signatures are robust to astrophysical
uncertainties, and will be easily detectable with second generation
interferometers. We also briefly show that decaying warm dark matter has a
negligible role in heating the IGM.
