  We present two complementary NuSTAR x-ray searches for keV-scale dark matter
decaying to mono-energetic photons in the Milky Way halo. In the first, we
utilize the known intensity pattern of unfocused stray light across the
detector planes -- the dominant source of photons from diffuse sources -- to
separate astrophysical emission from internal instrument backgrounds using
${\sim}$7-Ms/detector deep blank-sky exposures. In the second, we present an
updated parametric model of the full NuSTAR instrument background, allowing us
to leverage the statistical power of an independent ${\sim}$20-Ms/detector
stacked exposures spread across the sky. Finding no evidence of anomalous x-ray
lines using either method, we set limits on the active-sterile mixing angle
$\sin^2(2\theta)$ for sterile-neutrino masses 6--40 keV. The first key result
is that we strongly disfavor a ${\sim}$7-keV sterile neutrino decaying into a
3.5-keV photon. The second is that we derive leading limits on sterile
neutrinos with masses ${\sim}$15--18 keV and ${\sim}$25--40 keV, reaching or
extending below the Big Bang Nucleosynthesis limit. In combination with
previous results, the parameter space for the Neutrino Minimal Standard Model
($\nu$MSM) is now nearly closed.
