  We discuss a model where a mixed warm and hot keV neutrino dark matter rises
naturally. We arrange active and sterile neutrinos in the same $SU(3)_L$
multiplet, with the lightest sterile neutrino being dark matter. The other two
heavy sterile neutrinos, through their out-of-equilibrium decay, contribute
both to the dilution of dark matter density and its population, after
freeze-out. We show that this model features all ingredients to overcome the
overproduction of keV neutrino dark matter, and explore the phenomenological
implications for Big Bang Nucleosynthesis and the number of relativistic
degrees of freedom.
