A giant collection of galaxies immersed in hot gas about 250 million light years from Earth.
A new study of the Perseus galaxy cluster, shown in this image, and others using Chandra and XMM-Newton has revealed a mysterious X-ray signal in the data. This signal is represented in the circled data points in the inset, which is a plot of X-ray intensity as a function of X-ray energy. The signal is also seen in over 70 other galaxy clusters using XMM-Newton. This unidentified X-ray emission line – a spike of intensity centered on about 3.56 kiloelectron volts – requires further investigation to confirm both the signal’s existence and nature. One possibility is this signal is from the decay of sterile neutrinos, one proposed candidate to explain dark matter.