... Tonight's Sky . ... Gravitational wave search provides insights into galaxy evolution and mergers . ... Mysterious X-ray signal intrigues astronomers . One possibility is that the galaxy cluster X-rays are produced by the decay of sterile neutrinos, a type of particle that has been proposed as a candidate for dark matter. ... A mysterious X-ray signal has been found in a detailed study of galaxy clusters using NASA s Chandra X-ray Observatory and the European Space Agency s (ESA) XMM-Newton. ...