A mysterious X-ray signal from clusters of galaxies recently caused some excitement among astronomers: Does it perhaps originate from dark matter, which makes up around 80 percent of the matter in the universe, but which scientists have not yet been able to detect? In order to help answering this question, physicists at the Max Planck Institute for Nuclear Physics in Heidelberg checked an alternative explanation. Accordingly, the search for this form of matter, which is difficult to detect, must go on, as the mysterious X-ray signal seems to originate from highly charged sulfur ions that capture electrons from hydrogen atoms.
Around two years ago, the XMM-Newton X-ray satellite radioed data back to Earth which fired up great hopes with astrophysicists. It had picked up weak radiation from several galaxy clusters at an energy of around 3.5 kiloelectronvolts (keV) which the researchers were not immediately able to explain with the aid of the known X-ray spectra. Speculation quickly arose that they could be signals of decaying particles of dark matter – this would have been the first concrete trace of the long-sought form of matter. Hope was soon dampened, however: The regions in which XMM-Newton observed the X-ray radiation did not match the spatial distribution which astrophysical analyses predicted for dark matter.
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