An unidentified X-ray signature recently observed in nearby galaxies and galaxy clusters is not due to decay of dark matter, researchers report. The findings rule out previously proposed interpretations of dark matter particle physics. Dark matter (DM) constitutes more than 80% of the matter in the Universe and its gravitational pull is responsible for binding galaxies and galaxy clusters together. Despite its cosmological abundance and the well-established astrophysical evidence of its existence, little about the mysterious material is known, including which subatomic particles make up DM. Some models of potential DM particles predict that they might slowly decay into ordinary matter. If so, the process of dark matter decay would produce faint photon emissions detectable by X-ray telescopes. Recent X-ray observations of nearby galaxy clusters have detected an unidentified X-ray emission line at 3.5 kiloelectronvolts (keV), which has been interpreted by some as a signature of dark matter decay - particularly a hypothetical dark matter particle known as the sterile neutrino. If this is correct, DM surrounding our Galaxy should decay and produce a similar X-ray emission line, spread faintly across the entire night sky. Christopher Dessert and colleagues searched for the 3.5 keV signal within the ambient halo of the Milky Way using data from the European Space Agency's XMM-Newton space telescope. Dessert et al. analyzed blank-sky observations (parts of the sky away from large X-ray emitting regions) with a total exposure time of roughly a year, finding no evidence for the predicted 3.5 KeV line. According to the authors, the findings rule out the predicted signal strength by over an order of magnitude.