Dark matter, the enigmatic cosmic ghost, is everywhere, but remains unseen. Scientists believe it outweighs normal matter in the universe several times over, yet it’s so elusive that no telescope has ever captured it directly.
One promising dark matter candidate is a hypothetical particle called the axion. Some theories predict that if axions exist, they could transform into photons while crossing strong magnetic fields, releasing faint radio or X-ray signals. This is where magnetars, super-dense neutron stars with magnetic fields trillions of times stronger than Earth’s, come in.
Their extreme environments make them natural particle converters, potentially turning invisible axions into detectable light. For a long time, astronomers hoped that by monitoring magnetars for unusual signals, they might finally pin down dark matter’s true identity.
However, a team led by physicists in Lisbon has now revealed an unexpected complication. Their study suggests that much of this potential signal may vanish inside the magnetar’s own plasma environment, making the axions even harder to find than researchers once thought.
This actually brings us closer to finding axions
The project began with a simple question. The study authors asked themselves whether axions might interact with the charged-particle soup that surrounds magnetars. This plasma isn’t calm, but produces collective ripples known as plasmons.
The team realized that if axions mixed with these plasma waves, part of the axion’s energy might vanish into the star’s atmosphere before becoming detectable light. To test the idea, they built a mathematical model of what happens in a magnetar’s magnetosphere.
Their calculations showed that axions, instead of cleanly converting into photons, could lose much of their strength by coupling with plasmons. As a result, the outgoing radio signal would be far weaker than previous estimates.
“Imagine that previous researchers were listening for a specific note from a distant flute (the axion signal). They calculated how loud that note should be. Our work discovered that the flute has a leak, Hugo Terças, first author of the study, and an assistant professor at the Lisbon School of Engineering (ISEL), explained.
“Before the sound ever reaches us, some of the air (here, the axions) escapes through this leak into a different instrument that’s muted and can’t be heard (the plasmons). So, the note we’re trying to hear is much quieter than anyone calculated,” Terças added.
Moreover, the work does more than explain why astronomers may not have detected axions yet. It provides a framework to calculate exactly how much signal loss occurs under different plasma conditions.
That means future telescope searches can use more realistic expectations rather than chasing a note that may simply be too faint for current technology.
The discovery isn’t limited to dark matter
Interestingly, the same physics shows up far beyond dark matter research. In nuclear fusion experiments on Earth, engineers inject electromagnetic waves into donut-shaped reactors called tokamaks. The waves are absorbed by the plasma, turning into plasma oscillations that heat the fuel.
The process is almost identical to what the team predicts for axions around magnetars, showing how a cosmic puzzle connects back to clean-energy science at home.
“I believe the most exciting part of our work is how universal this underlying mechanism is. We discovered it in the extreme context of dark matter and magnetars, but it’s a fundamental process that pops up all over physics,” Terças said.
The researchers are now planning something bolder. They are moving the search from the sky into the lab. Their vision is to build a synthetic plasma, an engineered system that mimics the extreme environment of a magnetar’s atmosphere on a tabletop scale.
If successful, this setup could allow axions to reveal themselves under carefully controlled conditions, rather than waiting for the universe to send a signal our way.
“This would let us fine-tune the environment and essentially create the perfect conditions to coax axions into appearing through this conversion mechanism we’ve discovered. It’s a much more direct way to hunt for them,” Terças said.
The study is published in the journal Physical Review Letters.