  A sterile neutrino of ~keV mass is a well motivated dark matter candidate.
Its decay generates an X-ray line that offers a unique target for X-ray
telescopes. For the first time, we use the Gamma-ray Burst Monitor (GBM)
onboard the Fermi Gamma-Ray Space Telescope to search for sterile neutrino
decay lines; our analysis covers the energy range 10-25 keV (sterile neutrino
mass 20-50 keV), which is inaccessible to X-ray and gamma-ray satellites such
as Chandra, Suzaku, XMM-Newton, and INTEGRAL. The extremely wide field of view
of the GBM enables a large fraction of the Milky Way dark matter halo to be
probed. After implementing careful data cuts, we obtain ~53 days of full sky
observational data. We observe an excess of photons towards the Galactic
Center, as expected from astrophysical emission. We search for sterile neutrino
decay lines in the energy spectrum, and find no significant signal. From this,
we obtain upper limits on the sterile neutrino mixing angle as a function of
mass. In the sterile neutrino mass range 25-40 keV, we improve upon previous
upper limits by approximately an order of magnitude. Better understanding of
detector and astrophysical backgrounds, as well as detector response, will
further improve the sensitivity of a search with the GBM.
