Warm dark matter in two Higgs doublet models

We show that a neutral scalar field, sigma, of two-Higgs-doublet extensions of the Standard Model incorporating the seesaw mechanism for neutrino masses can be identified as a consistent warm dark matter candidate with a mass of order keV. The relic density of sigma is correctly reproduced by virtue of the late decay of a right-handed neutrino N participating in the seesaw mechanism. Constraints from cosmology determine the mass and lifetime of N to be M-N approximate to 25 GeV-20 TeV and tau(N) approximate to (10(-4) - 1) sec. These models can also explain the 3.5 keV X-ray anomaly in the extragalactic spectrum that has been recently reported in terms of the decay sigma -> gamma gamma. Future tests of these models at colliders and in astrophysical settings are outlined.