Massive sterile neutrinos as warm Dark Matter

We show that massive sterile neutrinos mixed with the ordinary ones may be produced in the early universe in the right amount to be natural warm dark matter particles. Their mass should be in the range 1-40 keV and the corresponding mixing angles sin$^2 2 \theta=10^{-8}$ to $10^{-11}$ for mixing with $\num$ or $\nut$, and mixing with $\nu_e$ is marginally allowed with mass about 30 keV and sin$^2\theta \approx 10^{-11}$.