University of Copenhagen physicists have developed a new observation tactic that uses the magnetic fields of galaxy clusters as a sort of natural particle accelerator in their hunt for the mysterious particle theorized to be at the heart of dark matter, which has long eluded detection.
Despite theoretically making up about 80% of the universe’s mass, dark matter has never been directly detected. Now, the University of Copenhagen researchers hope their work will help forge a path toward that goal.
The focus of the new research is axions, a theoretical elementary particle that, if finally found and measured to come within a specific range of low masses, may be one of the building blocks of cold dark matter.
A Natural Particle Accelerator
With a mass exceeding one quadrillion times that of the Sun, galaxy clusters are the heaviest structures in the universe. This makes them an interesting tool in discovering the first axions, a particle lighter than a single atom.
The University of Copenhagen scientists turned the cosmos into their laboratory by leveraging bright and distant black hole galaxies, focusing on the electromagnetic radiation emitted from their cores.
The team hypothesized that as the radiation passed through the extensive magnetic fields generated by galaxy clusters, it might form axions. Such a theoretical process should leave behind one major tell, according to the team’s work: random minuscule fluctuations in the data so small that they would likely be confused for random noise.
To look for such evidence, the team identified and observed 32 supermassive black holes located behind galaxy clusters from our perspective on Earth, and then collated that data.
“We looked at these black holes through clusters of galaxies,” explained co-author Oleg Ruchayskiy, Associate Professor at the Niels Bohr Institute of the University of Copenhagen. “Galaxy clusters are among the largest structures in the universe and reservoirs of enormous, widespread magnetic fields,” he says. “They act as a sort of prism through which some of the gamma rays in theory would turn into axions.”
Analyzing the Data
To the team’s surprise, they located just such a recognizable pattern as would be expected of an axion when looking at the data on a larger scale, across all of the black holes in the study.
“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,” Ruchayskiy said.
“It shows up like a unique step-like pattern that shows what this conversion could look like. We only see it as a hint of a signal in our data, but it is still very tantalizing and exciting. You could call it a cosmic whisper, now loud enough to hear,” he added.
The team’s work in discovering the pattern lends credence to the hypothetical axion’s existence, yet it is still a long way from a direct observation of dark matter. However, the work significantly advances an empirical search for dark matter by discovering one of its suspected building blocks.
Toward New Knowledge of Dark Matter
“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,” said co-author Lidiia Zadorozhna, a Marie Curie fellow at the Niels Bohr Institute.
The team’s innovation with their project is not limited to just this one particular application. They say that other types of electromagnetic radiation, such as X-rays, are also compatible with their new process, offering up a powerful new research tool with multiple applications.
“We are so excited, because it is not a one-time advancement. This method allows us to go beyond previous experimental limits and has opened a new path to studying these elusive particles,” Zadorozhna said.
“The technique can be repeated by us, by other groups, across a broad range of masses and energies,” Zadorozhna added. “That way, we can add more pieces to the puzzle of explaining dark matter.”
The paper, “Constraints on Axion-like Particles from Active Galactic Nuclei Seen Through Galaxy Clusters,” appeared in Nature Astronomy on August 15, 2025.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.