  We present a No-Go theorem for keV sterile neutrino Dark Matter: if sterile
neutrinos at the keV scale play the role of Dark Matter, they are typically
unstable and their decay produces an astrophysical monoenergetic X-ray line. It
turns out that the observational bound on this line is so strong that it
contradicts the existence of a quasi-degenerate spectrum of active neutrinos in
a seesaw type I framework where the Casas-Ibarra matrix R is real. This is the
case in particular for models without CP violation. We give a general proof of
this theorem. While the theorem (like every No-Go theorem) relies on certain
assumptions, the situation under which it applies is still sufficiently general
to lead to interesting consequences for keV neutrino model building. In fact,
depending on the outcome of the next generation experiments, one might be able
to rule out whole classes of models for keV sterile neutrinos.
