Dark matter could be made of sterile neutrinos, a low-mass particle that does not participate in the weak interaction, but which could be created through flavour oscillation. They would still interact via gravity.
Relic keV sterile neutrinos and reionisation
Authors: Peter L. Biermann, Alexander Kusenko
A sterile neutrino with mass of several keV can account for cosmological dark matter, as well as explain the observed velocities of pulsars. We show that X-rays produced by the decays of these relic sterile neutrinos can boost the production of molecular hydrogen, which can speed up the cooling of gas and the early star formation, which can, in turn, lead to a reionisation of the universe at a high enough redshift to be consistent with the WMAP results.
Read more (PDF)