Axion dark matter
Axions are hypothetical particles, far lighter than electrons, proposed in 1977 to fix a puzzle in physics. They may also be dark matter. How we hunt them.
By The Orbital Record editorial teamPublished Last reviewed
Axion dark matter is the idea that the invisible matter holding galaxies together is made of axions, hypothetical particles far lighter than electrons. Axions were proposed in 1977 and 1978 to fix a puzzle about the strong nuclear force, and turned out to have every property dark matter needs. None has been detected, but experiments are now sensitive enough to find the most likely kinds.
Axions are one of several candidates for dark matter, which makes up about 27% of the universe. For the evidence that dark matter exists at all, see our dark matter guide.
What is an axion?
An axion is a proposed elementary particle: electrically neutral, extremely light and almost never interacting with ordinary matter. It came from particle physics, not astronomy.
The puzzle it was invented to solve is called the strong CP problem. The theory of the strong force, which binds quarks inside protons and neutrons, allows a term that would make the force behave differently for matter and its mirror image. If that term were large, the neutron would have a measurable electric dipole, a slight separation of positive and negative charge. Experiments find none, so the term must be tiny or zero, with no reason given by the theory.
In 1977 Roberto Peccei and Helen Quinn proposed a mechanism that sets the term to zero naturally. In 1978 Steven Weinberg and Frank Wilczek, in separate papers in Physical Review Letters, showed that the mechanism implies a new particle. Wilczek named it, as he later explained in his Nobel lecture: “I named them after a laundry detergent, since they clean up a problem with an axial current.”
Why axions could be dark matter
Axions were not designed to be dark matter, which is why physicists take the idea seriously: it arrived as a bonus. As the Nobel lecture put it, they are “a leading candidate to provide cosmological dark matter.”
Axions produced in the early universe would be:
- Cold, moving slowly, which is what cosmic structure needs (cold dark matter).
- Invisible, because they barely interact with light.
- Stable, lasting far longer than the age of the universe.
- Extremely numerous, because each is so light that vast numbers are needed to add up to the mass of dark matter.
Because each axion is so light, axion dark matter would behave less like a swarm of billiard balls and more like a wave filling the galaxy. That is the feature experiments exploit.
Axions vs WIMPs
| Axions | WIMPs | |
|---|---|---|
| Proposed to solve | The strong CP problem of the strong force | Dark matter abundance (the “WIMP miracle”) |
| Mass | Far below an electron’s; searches mostly target millionths of an electronvolt | Roughly a proton’s mass to thousands of times more |
| Behaves like | A wave filling space | Individual particles |
| How we search | Convert axions into microwaves or X-rays with strong magnets | Watch for recoils of atoms in underground detectors |
| Main experiments | ADMX, HAYSTAC, MADMAX, CAST, BabyIAXO | LZ and other liquid xenon detectors |
| Status, October 2026 | Not found; benchmark models excluded in narrow mass ranges | Not found; LZ set world-leading limits in December 2025 |
The WIMP searches are described in our pillar and in the paper on LZ’s first dark matter search. In 2026 LZ also reported a single unexplained event at low significance, which the team stressed was not a claim of dark matter (LZ’s single-event hint).
How do scientists search for axion dark matter?
Axions should occasionally turn into photons inside a strong magnetic field. Each experiment uses that conversion in a different way:
- Haloscopes look for axions from the Milky Way’s dark matter halo. The Axion Dark Matter eXperiment (ADMX), hosted by the University of Washington with Fermilab as lead Department of Energy laboratory, places a microwave cavity inside a superconducting magnet, cooled to below a kelvin. If axions are present, a faint signal appears when the cavity is tuned to the frequency matching the axion’s mass. In 2018 ADMX became the first experiment sensitive to plausible “invisible axion” models, and by 2020 it had excluded benchmark axions between 2.66 and 3.31 millionths of an electronvolt (μeV). Results published in 2025 extended the search to higher frequencies, up to 1.31 gigahertz.
- Squeezed-light haloscopes. HAYSTAC used quantum squeezing to double its search speed and reported no signal near 17 μeV in Nature in 2021.
- Dielectric haloscopes stack disks to boost the signal at higher masses. A MADMAX prototype, three sapphire disks and a mirror in a magnet at CERN, published the first axion search of this kind in July 2025, at masses well above ADMX’s range.
- Helioscopes look for axions made in the Sun. The CERN Axion Solar Telescope (CAST) pointed a 9-tesla LHC test magnet at the Sun from 2003 to 2015 and set a leading limit on how strongly axions couple to light. BabyIAXO, a larger successor planned at DESY in Germany, is designed to go further.
The difficulty is the unknown mass. Each experiment can only scan a narrow slice, slowly, the way you would search an enormous radio dial for one faint station. False alarms happen: in July 2026 a haloscope team published that a candidate excess near 1.036 gigahertz did not persist under further tests.
Fuzzy dark matter: the ultralight extreme
Some physicists consider axion-like particles far lighter still. In “fuzzy” dark matter, as described by Hui, Ostriker, Tremaine and Witten (Physical Review D, 2017), the particle mass is around 10-22 electronvolts, and its quantum wavelength is about 1 kiloparsec, roughly 3,000 light-years. Dark matter would then behave like a wave on the scale of small galaxies, forming smooth cores at their centers. Astronomers test the idea with dwarf galaxies and thin stellar streams.
What would finding an axion mean?
A detection would solve two problems at once: the nature of dark matter and the strong CP problem. It would also pin down the particle’s mass precisely, because haloscopes measure a frequency. Until then, axions remain a well-motivated candidate competing with WIMPs, sterile neutrinos and primordial black holes, compared side by side in our dark matter guide. For how dark matter differs from the force speeding up cosmic expansion, see dark matter vs dark energy.
Questions people ask
What is an axion?
An axion is a hypothetical elementary particle, predicted in 1978 by Steven Weinberg and Frank Wilczek, that would solve a long-standing puzzle about the strong nuclear force called the strong CP problem. Axions would be extremely light, far lighter than electrons, electrically neutral and very weakly interacting, which also makes them a natural dark matter candidate.
Are axions dark matter?
They could be, but nobody has detected one. Axions produced in the early universe would form a cold, invisible sea of particles with exactly the properties dark matter needs. Experiments have ruled out benchmark axion models in some narrow mass ranges, but each search covers only a thin slice of possible masses.
Why are axions called axions?
Frank Wilczek named them after a brand of laundry detergent. In his Nobel lecture he explained that the name fit because the particles 'clean up a problem with an axial current', a technical feature of the strong force theory. Steven Weinberg had independently predicted the same particle in 1978.
What is the difference between axions and WIMPs?
Both are proposed dark matter particles, but their masses differ enormously. WIMPs would weigh roughly as much as a proton or more and are hunted through collisions in underground detectors. Axions would be hundreds of billions of times lighter than an electron and behave more like a wave, so experiments try to turn them into radio waves instead.
How do scientists search for axions?
Mostly with haloscopes, such as ADMX, which place a tuned microwave cavity inside a powerful magnet. If dark matter axions pass through, a few should convert into microwave photons at a frequency set by the axion's mass. Scientists slowly tune through frequencies, like searching a radio dial for a faint station.
Has an axion been detected?
No. Several experiments have reported candidate signals that disappeared under scrutiny. In July 2026, for example, a haloscope team published that an excess near 1.036 gigahertz did not persist in follow-up tests and was not an axion. As of October 2026, no axion has been detected in any experiment.
What is fuzzy dark matter?
Fuzzy dark matter is the idea that dark matter consists of extraordinarily light particles, around 10−22 electronvolts, often axion-like. At that mass their quantum wavelength would be about 1 kiloparsec, roughly 3,000 light-years, so dark matter would behave like a wave on galactic scales and smooth out the centers of small galaxies.