One of the possible explanations for the dark matter needed in the standard
cosmological model is so-called warm dark matter, in the form of right-handed
("sterile") neutrinos with a mass in the keV range. I describe how various
properties of QCD at temperatures of a few hundred MeV play an important role
in the theoretical computations that are needed for consolidating or falsifying
this scenario. In particular the points where lattice QCD could help are
underlined.Comment: 15 pages. Updated version of plenary talk at International Symposium
on Lattice Field Theory, July 23-28, 2006, Tucson, Arizona, US
