  Sterile neutrinos with masses in the keV range are well-motivated extensions
to the Standard Model that could explain the observed neutrino masses while
also making up the dark matter (DM) of the Universe. If sterile neutrinos are
DM then they may slowly decay into active neutrinos and photons, giving rise to
the possibility of their detection through narrow spectral features in
astrophysical X-ray data sets. In this work, we perform the most sensitive
search to date for this and other decaying DM scenarios across the mass range
from 5 to 16 keV using archival XMM-Newton data. We reduce 547 Ms of data from
both the MOS and PN instruments using observations taken across the full sky
and then use this data to search for evidence of DM decay in the ambient halo
of the Milky Way. We determine the instrumental and astrophysical baselines
with data taken far away from the Galactic Center, and use Gaussian Process
modeling to capture additional continuum background contributions. No evidence
is found for unassociated X-ray lines, leading us to produce the strongest
constraints to date on decaying DM in this mass range.
