Opening a New Window for Warm Dark Matter

We explore the range of parameters for dark-matter sterile neutrinos in an extention of the Minimal Standard Model by three singlet fermions with masses below the electroweak scale (the $\nu$MSM). This simple model, which can explain a wide range of phenomena, including neutrino oscillations, baryogenesis, the pulsar velocities, and the early reionization, allows for the dark-matter sterile neutrinos to have masses as small as 0.5 keV. The presence of two heavier sterile neutrinos and the possibility of entropy production in their decays broadens the allowed range of parameters for the dark-matter sterile neutrinos (or other types of dark matter, for example, the gravitino). Of particular interest is the possibility of large mixing angles between active and sterile neutrinos, which may be accessible to laboratory experiments.