Mysterious X-ray signal could be dark matter candidate

Scientists have detected a possible signal of the elusive dark matter, the mysterious, invisible stuff that makes up most of the matter in the universe.
A mysterious X-ray signal found in a detailed study of galaxy clusters may be produced by the decay of sterile neutrinos, a type of particle that has been proposed as a candidate for dark matter, scientists said.
While holding exciting potential, these results must be confirmed with additional data to rule out other explanations and determine whether it is plausible that dark matter has been observed, researchers said.
Astronomers think dark matter constitutes 85 per cent of the matter in the universe, but does not emit or absorb light like "normal" matter such as protons, neutrons and electrons that make up the familiar elements observed in planets, stars, and galaxies.

Because of this, scientists must use indirect methods to search for clues about dark matter.
The latest results from NASA's Chandra X-ray Observatory and ESA's XMM-Newton consist of an unidentified X-ray emission line, that is, a spike of intensity at a very specific wavelength of X-ray light.

Astronomers detected this emission line in the Perseus galaxy cluster. They also found the line in a combined study of 73 other galaxy clusters with XMM-Newton.
"We know that the dark matter explanation is a long shot, but the pay-off would be huge if we're right," said Esra Bulbul of the Harvard-Smithsonian Centre for Astrophysics (CfA) in Cambridge, who led the study.

"So we're going to keep testing this interpretation and see where it takes us," said Bulbul.
The authors suggest this emission line could be a signature from the decay of a "sterile neutrino."
Sterile neutrinos are a hypothetical type of neutrino that is predicted to interact with normal matter only via gravity. Some scientists have proposed that sterile neutrinos may at least partially explain dark matter.
"We have a lot of work to do before we can claim, with any confidence, that we've found sterile neutrinos," said Maxim Markevitch, a co-author from NASA's Goddard Space Flight Centre in Greenbelt, Maryland.

"But just the possibility of finding them has us very excited," said Markevitch.
One source of uncertainty is that the detection of this emission line is pushing the capabilities of the two observatories in terms of sensitivity.
Also, there may be explanations other than sterile neutrinos if this X-ray emission line is deemed to be real.
There are ways that normal matter in the cluster could have produced the line, although the team's analysis suggested that all of these would involve unlikely changes to our understanding of physical conditions in the galaxy cluster or the details of the atomic physics of extremely hot gases.

The research was published in The Astrophysical Journal.