  One of the possible methods to distinguish among various dark matter
candidates is to study the effects of dark matter decays. We consider four
different dark matter candidates (light dark matter, gravitinos, neutralinos
and sterile neutrinos), for each of them deriving the decaying/annihilation
rate, the influence on reionization, matter temperature and CMB spectra. We
find that light dark matter particles (1-10 MeV) and sterile neutrinos (2-8
keV) can be sources of partial early reionization (z<~100). However, their
integrated contribution to Thomson optical depth is small (<~0.01) with respect
to the three year WMAP results (tau_e=0.09+/-0.03). Finally, they can
significantly affect the behavior of matter temperature. On the contrary,
effects of heavy dark matter candidates (gravitinos and neutralinos) on
reionization and heating are minimal. All the considered dark matter particles
have completely negligible effects on the CMB spectra.
