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An unusual gamma-ray signal could be the most direct evidence of dark matter yet. However, the signal could also be the result of a telescope anomaly or produced by something even stranger than dark matter.

Dark matter forms the bones of much of our universe, driving how galaxies cluster, for example. It also far outweighs all the visible matter, yet for decades researchers have puzzled over what exactly it is.

Part of the problem is that they cannot currently observe dark matter directly. Instead, they can only register its effect on its surroundings. A more direct line of investigation would be to detect particles produced when dark matter collides and annihilates with itself, which some theories suggest ought to happen. Now, at Guangxi University in China and his colleagues identified a gamma-ray signal that may be the result of this process.

They found the signal after analysing 15.5 years of data from the Fermi Gamma-ray Space Telescope (FGST), looking at the Virgo, Fornax, and Ophiuchus galaxy clusters specifically. The researchers chose these galaxy clusters because they are known to contain large halos of dark matter, says Liang. The gamma-ray signal the team found there was shaped like a spike or a line, analogous to a burst of light of only one colour, which some theories predict is the result of dark matter annihilating itself.

“Discovering a sharp gamma-ray line would be the ultimate ‘smoking gun’ evidence proving the existence of dark-matter particles and revealing their properties in particle physics,” says team member at the Chinese Academy of Sciences. Based on their statistical analysis of the data, the researchers estimated that there is less than a 1-in-10,000 chance that this signal is random cosmic noise or a purely coincidental pattern.

Yet, several questions remain before the team can declare a definitive discovery. Detecting dark-matter signatures in gamma rays has historically been difficult because the signals tend to be weak and can be confused with telescope errors. One promising gamma-ray , for example, proved to be exactly that. , also at the Chinese Academy of Sciences, who worked on the new study, says the team members performed a large number of tests to confirm the veracity of their signal, but some possibility of instrument error remains.

Moreover, while the new signal is clear when looking at the three galaxy clusters, it seems to vanish closer to the centre of our galaxy, where dark matter is dense, so it should also be annihilating itself and creating gamma rays.

“The fact that the signal lights up in distant clusters but remains silent in our own cosmic backyard is deeply peculiar,” says Liang. “If this is truly from dark matter, it suggests the particles must interact in a much more sophisticated way than conventional theories predict.” Alternatively, the signal could be coming from some even more rare or novel phenomenon, such as ultra-fast particle winds from exotic magnetized neutron stars.

“I find the result intriguing, but I would be quite cautious about interpreting it as evidence for dark matter,” says at Liverpool University in the UK. He says that looking at only one galaxy cluster instead of all three together increases the chances that the signal is a product of noise or a random pattern. And he is also worried about how unconventional a dark-matter process would have to be to produce this gamma-ray spike far away from us but not close to the centre of our galaxy. “This puts the standard dark-matter interpretation under considerable tension,” he says.

The team hopes that future telescope missions will add clarity to the situation, for instance the Very Large Area Gamma-ray Space Telescope, which was proposed several years ago and could collect more gamma rays with better resolution. By 2040, FGST will also have doubled its data set, which could further elucidate the nature of the odd gamma-ray spike, says Fan.

“If future telescopes confirm that this signal is genuine, it would represent a historic breakthrough. It would immediately reveal the mass of the elusive dark-matter particle and give physicists a concrete target to build the next generation of fundamental particle theories,” says Shen. “At the same time, it would force us to thoroughly rewrite our standard textbook models of dark matter.”