Standard Model Neutrinos as Warm Dark Matter
  Standard Model neutrinos are not usually considered plausible dark matter candidates because the usual treatment of their decoupling in the early universe implies that their mass must be sufficiently small to make them ``hot'' dark matter. In this paper we show that decoupling of Standard Model neutrinos in low reheat models may result in neutrino densities very much less than usually assumed, and thus their mass may be in the keV range. Standard Model neutrinos may therefore be warm dark matter candidates. 
Gian F. Giudice, Edward W. Kolb, Antonio Riotto, Dmitry V. Semikoz,   Igor I. Tkachev