Dark matter has remained one of the biggest riddles in modern physics. Astronomers know it must be there as it outweighs visible matter several times over and shapes the growth of galaxies. However, its actual form has never been pinned down.
One long-suspected dark matter candidate is the axion, a particle so feather-light and faintly interacting that it has slipped through every scientific search so far. Even the most advanced laboratories have failed to detect its presence.
Now, researchers at the University of Copenhagen have tried something unusual. Instead of looking for axions on Earth, they turned the largest objects in the cosmos into their experiment. Their findings reveal a signal that looks strikingly like the kind of fingerprint axions would leave behind.
“Astrophysical sites can serve as alternative laboratories for particle physics. In particular, they offer unique opportunities to study hypothetical particles that interact minimally with known forms of matter,” the study authors note.
Looking for tiny axions in galactic giants
The researchers’ idea was to use galaxy clusters, giant knots of hundreds of galaxies bound together by gravity. These clusters are not only a quadrillion times heavier than the Sun, but they also host magnetic fields stretching across intergalactic space.
Axions, on the other hand, are one of the lightest particles in the universe. So then, how come one can pinpoint them in something as big as a galaxy cluster?
According to the study authors, light passing through the magnetic fields of clusters could occasionally convert into axions. The evidence of this event could appear in radiation coming from bright sources lying behind the clusters.
The researchers chose 32 such sources, including active galaxies powered by supermassive black holes, each producing powerful streams of high-energy light. As this light traveled across the vast cluster of magnetic fields, some of it might have briefly turned into axions and back into photons, leaving tiny irregularities in the data.
The difficulty is that each single observation looks like meaningless static. However, something surprising happened when the team combined the data from all 32 black holes. What previously resembled random noise began to align into a clear step-like shape, the kind of pattern models predict if photon–axion conversion has occurred.
“Normally, the signal from such particles is unpredictable and appears as random noise. But we realized that by combining data from many different sources, we had transformed all that noise into a clear, recognizable pattern. You could call it a cosmic whisper, now loud enough to hear,” Oleg Ruchayskiy, senior study author and a professor at the University of Copenhagen, said.
Not the final result, but a strong hint
The Copenhagen results don’t prove that axions exist, but they do bring us closer. By ruling out wide ranges of possible properties, the study has narrowed down where axions could be hiding.
“This method has greatly increased what we know about axions. It essentially enabled us to map a large area that we know does not contain the axion, which narrows down the space where it can be found,” Lidiia Zadorozhna, one of the lead authors and a postdoc researcher at the University of Copenhagen, said.
More importantly, the method can be applied again, not just with gamma rays but also with X-rays or other parts of the spectrum, and by research groups worldwide.
If future studies confirm the signal, the consequences would be profound. Axions could finally explain the invisible mass holding galaxies together and provide an answer to the nearly 100-year-old dark matter mystery.
The study is published in the journal Nature Astronomy.