Prof. Abazajian wrote a general physics perspective viewpoint for Physical Review Letters on the two groups that have detected an unidentified 3.5 keV X-ray line that is consistent with dark matter decay. One group, Bulbul et al. (2014), has detected the line in two independent XMM-Newton X-ray Space Telescope stacked cluster observations, and in the Perseus cluster with Chandra X-ray Space Telescope observations. Another group, Boyarsky et al. (2014), detected the line in the Andromeda Galaxy and the Perseus cluster with XMM-Newton observations.