  We point out a serendipitous link between warm dark matter (WDM) models for
structure formation on the one hand and the high sensitivity energy range (1-10
keV) for x-ray photon detection on the Chandra and XMM-Newton observatories on
the other. This fortuitous match may provide either a direct detection of the
dark matter or exclusion of many candidates. We estimate expected x-ray fluxes
from field galaxies and clusters of galaxies if the dark matter halos of these
objects are composed of WDM candidate particles with rest masses in the
structure formation-preferred range (~1 keV to ~20 keV) and with small
radiative decay branches. Existing observations lead us to conclude that for
singlet neutrinos (possessing a very small mixing with active neutrinos) to be
a viable WDM candidate they must have rest masses < 5 keV in the zero lepton
number production mode. Future deeper observations may detect or exclude the
entire parameter range for the zero lepton number case, perhaps restricting the
viability of singlet neutrino WDM models to those where singlet production is
driven by a significant lepton number. The Constellation X project has the
capability to detect/exclude singlet neutrino WDM for lepton number values up
to 10% of the photon number. We also consider diffuse x-ray background
constraints on these scenarios. These same x-ray observations additionally may
constrain parameters of active neutrino and gravitino WDM candidates.
