  The seesaw mechanism in models with extra dimensions is shown to be
generically consistent with a broad range of Majorana masses. The resulting
democracy of scales implies that the seesaw mechanism can naturally explain the
smallness of neutrino masses for an arbitrarily small right-handed neutrino
mass. If the scales of the seesaw parameters are split, with two right-handed
neutrinos at a high scale and one at a keV scale, one can explain the
matter-antimatter asymmetry of the universe, as well as dark matter. The dark
matter candidate, a sterile right-handed neutrino with mass of several keV, can
account for the observed pulsar velocities and for the recent data from Chandra
X-ray Observatory, which suggest the existence of a 5 keV sterile right-handed
neutrino.
