Probing the nature of dark matter with cosmic x rays: Constraints from dark blobs and grating spectra of galaxy clusters

Gravitational lensing observations of galaxy clusters have identified dark matter blobs with remarkably low baryonic content. We use such a blob to probe the particle nature of dark matter with x-ray observations. From these observations we improve the most conservative constraints from the Milky Way halo on a particular dark matter candidate, the sterile neutrino, by an order of magnitude. We also study high resolution x-ray grating spectra of a cluster of galaxies. Based on these conservative constraints obtained from cosmic x-ray observations alone, the low mass (m(s)less than or similar to 10 keV) and low mixing angle (sin(2)(2 theta)less than or similar to 10(-6)) sterile neutrino is still a viable dark matter candidate.