Long-exposure NuSTAR constraints on decaying dark matter in the Galactic halo

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