Dark matter is one of the biggest mysteries in our universe. We think it makes up about 85 percent of all matter, but because it doesn’t emit or absorb light like ordinary matter, we can’t see or measure it.
But now, astronomers using ESA’s XMM-Newton and NASA’s Chandra X-ray observatories have detected an unidentified X-ray emission line – a spike of intensity at a very specific wavelength of X-ray light – in the Andromeda galaxy, the Perseus galaxy cluster and 73 other galaxy clusters.
One intriguing possibility is that these X-rays are produced by the decay of sterile neutrinos, a type of particle that has been proposed as a candidate for dark matter.
While more data is necessary to support the claims, researchers believe the X-ray signal, if accurate, could help explain dark matter.
Perseus is one of the most massive objects in the universe, containing thousands of galaxies in a cloud of superheated hydrogen gas. At over 10 million degrees Celsius, that gas is hot enough to emit X-rays. The image of Perseus above combines 17 days’ worth of data taken over a decade.
The new findings consist of an unidentified X-ray emission line: a spike in intensity at a very specific wavelength of X-ray light. In this case, it was centered on about 3.56 kiloelectron volts (keV). Lead author of the Perseus study Esra Bulbul says that a possible explanation for the X-ray emission line is sterile neutrino decay, which is a candidate for dark matter.
Sterile neutrinos are a hypothetical type of neutrino that doesn’t interact through weak force; they’re predicted to interact with “normal” matter only through gravity.
Some scientists have proposed that such neutrinos may at least partially explain dark matter.
“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 Center for Astrophysics in Cambridge. “So we’re going to keep testing this interpretation and see where it takes us.”
“We have a lot of work to do before we can claim, with any confidence, that we’ve found sterile neutrinos. But just the possibility of finding them has us very excited,” said Dr Maxim Markevitch from NASA’s Goddard Space Flight Center in Greenbelt, Maryland, who is a co-author of the ApJ paper.
There are a few reasons astronomers are being cautious in getting too excited over having potentially found traces of dark matter. For one, the emission line Chandra and XMM-Newton observed is right at the edge of both observatories’ sensitivity, meaning it might not be a real line. If the emission line turns out to be real, and even if this line is determined to be real, there may be other explanations.
It’s unlikely, but normal matter could produce the observed signal, though if this were the case it would impact what we know about the physical characteristics of galaxy clusters and atomic physics in hot gases. All this is to say there’s a lot more work to be done. Their next step is to combine data from Chandra with JAXA’s Suzaku to see if they can find the same X-ray signal.