  We study the case of DM self annihilation, in order to assess its importance
as an energy injection mechanism, to the IGM in general, and to the medium
within particular DM haloes. We consider thermal relic WIMP particles with
masses of 10GeV and 1TeV and we analyse in detail the clustering properties of
DM in a $\Lambda$CDM 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 SMBH. 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 ~ 10, for the case of a 1TeV
WIMP, the ionization fraction could be raised to $6 \times 10^{-4}$ and the
temperature to 10K, and in the case of a 10GeV WIMP, the IGM temperature could
be raised to 200K and the ionization fraction to $8 \times 10^{-3}$. 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
($\delta$Tb), we conclude that a thermal relic WIMP with mass of 1TeV is not
likely to be detected from the global signal alone, except perhaps at the 1-3mK
level in the frequency range 30MHz < $\nu$ < 35MHz corresponding to 40 < z <
46. However, a 10GeV mass WIMP may be detectable at the 1-3mK level in the
frequency range 55MHz < $\nu$ < 119MHz corresponding to 11 < z < 25, and at the
1-10mK level in the frequency range 30MHz < $\nu$ < 40MHz corresponding to 35 <
z < 46.
