  Observations of nearby galaxies and galaxy clusters have reported an
unexpected X-ray emission line around 3.5 kilo-electron volts (keV). Proposals
to explain this line include decaying dark matter$-$in particular, that the
decay of sterile neutrinos with a mass around 7 keV could match the available
data. If this interpretation is correct, the 3.5 keV line should also be
emitted by dark matter in the halo of the Milky Way. We used more than 30
megaseconds of XMM-Newton (X-ray Multi-Mirror Mission) blank-sky observations
to test this hypothesis, finding no evidence of the 3.5-keV line emission from
the Milky Way halo. We set an upper limit on the decay rate of dark matter in
this mass range, which is inconsistent with the possibility that the 3.5-keV
line originates from dark matter decay.
