Sterile neutrino dark matter in $B-L$ extension of the standard model   and galactic 511 keV line
  Sterile right-handed neutrinos can be naturally embedded in a low scale gauged $U(1)_{B-L}$ extension of the standard model. We show that, within a low reheating scenario, such a neutrino is an interesting candidate for dark matter. We emphasize that if the neutrino mass is of order of MeV, then it accounts for the measured dark matter relic density and also accommodates the observed flux of 511 keV photons from the galactic bulge. 
Shaaban Khalil and Osamu Seto