  In usual particle models, sterile neutrinos can account for the dark matter
of the Universe only if they have masses in the keV range and are warm dark
matter. Stringent cosmological and astrophysical bounds, in particular imposed
by X-ray observations, apply to them. We point out that in a particular
variation of the inert doublet model, sterile neutrinos can account for the
dark matter in the Universe and may be either cold or warm dark matter
candidates, even for masses much larger than the keV range. These Inert-Sterile
neutrinos, produced non-thermally in the early Universe, would be stable and
have very small couplings to Standard Model particles, rendering very difficult
their detection in either direct or indirect dark matter searches. They could
be, in principle, revealed in colliders by discovering other particles in the
model.
