Constraints on sterile neutrino as a dark matter candidate from the   diffuse X-ray background
  Sterile neutrinos with masses in the keV range are viable candidates for the warm dark matter. We analyze existing data for the extragalactic diffuse X-ray background for signatures of sterile neutrino decay. The absence of detectable signal within current uncertainties of background measurements puts model-independent constraints on allowed values of sterile neutrino mass and mixing angle, which we present in this work. 
A. Boyarsky, A. Neronov, O.Ruchayskiy, M.Shaposhnikov