Flashes of X-rays from crowded galaxy clusters could be the long-awaited sign that we have found particles of dark matter – the elusive substance thought to make up the bulk of all matter in the universe.
If the results stand up, dark matter would consist of ghostly particles called "sterile" neutrinos. These tantalising particles would be the first kind found beyond the standard set known to science.
Dark matter interacts with ordinary matter via gravity but otherwise scarcely makes itself known. Physicists think its mass could be tied up in an unknown particle. The leading theoretical candidate is a weakly interacting massive particle (WIMP), but our best detectors have yet to yield a confirmed sighting.
Neutrinos also seem like good candidates, as they are long-lived and notoriously aloof when it comes to interacting with other particles. But the three known types of neutrinos are not massive enough to account for all of the universe's dark matter, so theorists proposed a fourth neutrino that would interact even less with regular matter. If this sterile neutrino is heavy enough – about 10 kiloelectronvolts (keV) – it could account for all dark matter, while lighter versions might solve other cosmic conundrums (see "Ghost in the galactic machine", below).
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