  We show that, within the see-saw mechanism, an almost decoupled RH neutrino
species N_DM with mass M_DM \gtrsim 100 GeV can play the role of Dark Matter
(DM). The N_DM's can be produced from non-adiabatic conversions of thermalized
(source) RH neutrinos with mass M_S lower than M_DM. This is possible if a
non-renormalizable operator is added to the minimal type I see-saw lagrangian.
The observed DM abundance can be reproduced for M_DM \delta^{1/4} \sim 10^-13
\Lambda_eff \xi, where \Lambda_eff is a very high energy new physics scale,
\delta \equiv (M_DM-M_S)/M_DM and \xi \lesssim 1 is a parameter determined by
the RH neutrino couplings.
