Is it singing the song of the sterile neutrino? (Image: NASA, ESA, CFHT, CXO, M. J. Jee (University of California, Davis), and A. Mahdavi (SFSU))

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).

This particle would be virtually impossible to detect directly, so we must catch it decaying into other particles that are easier to observe – X-ray photons and ordinary neutrinos. Two independent groups used archival data from X-ray telescopes to search for these decay products. They focused their searches on galaxy clusters, which should be rich in dark matter.

Esra Bulbul at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, and her colleagues used observations of 73 galaxy clusters from the European Space Agency’s XMM-Newton space telescope and NASA’s Chandra X-ray telescope. The other team, led by Alexey Boyarsky at Leiden University in the Netherlands, used XMM-Newton observations of the Perseus galaxy cluster and the nearby Andromeda galaxy.

Both teams saw X-ray emissions at energies of about 3.5 keV. That could correspond to the decay of a sterile neutrino with a mass of about 7 keV. There is no other obvious explanation for what could be producing the signal: nothing else that we know of emits X-rays at that specific energy. Both teams were able to rule out noise from the telescopes.

“We tested every single scenario we could come up with and eliminated things very carefully,” says Bulbul. “The upshot is we just don’t know what this is. The most exciting explanation is the decay signature of sterile neutrinos.”

Signs of sterile neutrinos have popped up and been knocked down before, so the researchers are being cautious.

“It’s intriguing. There’s a consistent picture for it being dark matter,” says Kevork Abazajian at the University of California, Irvine, who was not involved in either study. “But I think confirming it would really require deeper observations of other things.” His wish list includes checking if the signal shows up in other objects that should have lots of dark matter – such as dwarf galaxies – and finding out if the signal is stronger in the centres of clusters, where dark matter should clump more.

Journal references: Bulbul et al, arxiv.org/abs/1402.2301 and Boyarsky et al, arxiv.org/abs/1402.4119v1

Ghost in the galactic machine If they exist, sterile neutrinos could explain dark matter. But even if they cannot account for it all, finding them may solve another mystery: where are the universe’s “missing” galaxies? Maps of the first light emitted after the big bang have been used to measure how fast the entire universe is expanding and how readily regular matter clumps up. However, these data disagree with experiments that measure the same values in the local universe: the maps say the universe is expanding more slowly, and that there should be almost three times as many galaxy clusters as we observe. If sterile neutrinos existed in the early universe, they could resolve both problems, says Mark Wyman of New York University. His models show that a sterile neutrino with a mass of less than 1 electronvolt would speed up the map-based values for the universe’s expansion. In addition, the neutrinos’ reluctance to interact would let them escape from early clumps of matter, meaning less mass gets left behind to pile up and build galaxies. Wyman’s sterile neutrino is too light to account for dark matter, but it might be related to heavier sterile neutrinos, signs of which may be appearing in X-ray data (see main story). “It would be awesome if these sterile neutrinos were somehow related, but as yet no one has threaded all of those needles simultaneously,” he says. Journal reference: Physical Review Letters, DOI: 10.1103/PhysRevLett.112.051302

This article will appear in print under the headline “Hint of dark matter made of neutrinos”