We study the case of dark matter (DM) self-annihilation, in order to assess its importance as an energy injection mechanism, to the intergalactic medium (IGM) in general, and to the medium within particular DM haloes. We consider thermal relic WIMP (weakly interacting massive particle) particles with masses of 10GeV and 1 TeV, and we analyse in detail the clustering properties of DM in a Lambda cold dark matter cosmology, on all hierarchy levels, from haloes and their mass function, to subhaloes and the DM density profiles within them, considering adiabatic contraction by the presence of a supermassive black hole. We then compute the corresponding energy output, concluding that DM annihilation does not constitute an important feedback mechanism. We also calculate the effects that DM annihilation has on the IGM temperature and ionization fraction, and we find that assuming maximal energy absorption, at z similar to 10, for the case of a 1 TeV WIMP, the ionization fraction could be raised to 6 x 10(-4) and the temperature to 10 K, and in the case of a 10 GeV WIMP, the IGM temperature could be raised to 200 K and the ionization fraction to 8 x 10(-3). We conclude that DM annihilations cannot be regarded as an alternative reionization scenario. Regarding the detectability of the WIMP through the modifications to the 21 cm differential brightness temperature signal (delta T-b), we conclude that a thermal relic WIMP with mass of 1 TeV is not likely to be detected from the global signal alone, except perhaps at the 1-3 mK level in the frequency range 30 < nu < 35 MHz corresponding to 40 < z < 46. However, a 10 GeV mass WIMP may be detectable at the 1-3mK level in the frequency range 55 < nu < 119 MHz corresponding to 11< z < 25, and at the 1-10 mK level in the frequency range 30< nu < 40 MHz corresponding to 35 < z < 46.