Here’s what you’ll learn when you read this story:

• Several theories have attempted to explain excess far-ultraviolet light present in the Milky Way, but none have been satisfactory.
• Now, researchers have put forth a hypothesis that axion quark nuggets of dark matter may be colliding with visible matter, releasing ultraviolet light.
• This work could have implications for understanding light emissions in the early universe.

Elusive but everywhere, dark matter is thought to make up about 27% of the universe—but not one molecule of it has been directly detected so far. It’s completely invisible. The only way to detect dark matter is through the gravitational effects it has on baryonic or visible matter.

Well… that, and the phantom glow it seems to be emitting.

Dark matter may come in more than just one form. One hypothesis suggests that some of it exists as axion quark nuggets (AQNs), which are ultra-dense objects made of quarks and linked to axions (still-hypothetical ultralight particles). AQNs would behave gravitationally like cold dark matter—the invisible, slow-moving matter thought to help shape galaxies. But unlike many dark matter candidates, AQNs could also leave observable traces in the form of electromagnetic radiation across a range of wavelengths when they interact with ordinary matter. And if some AQNs are made of antimatter, those encounters would be even more dramatic, triggering annihilation events that would convert mass into bursts of light. In principle, that process could help explain the Milky Way’s unexplained excess of far-ultraviolet radiation.

Michael Sekatchev (an astrophysicist at the University of California, Berkeley) wanted to test whether antimatter AQNs could account for the ultraviolet glow that scientists have noticed is left over after all known sources have been factored out. About a decade ago, NASA’s Galaxy Evolution Explorer (GALEX) used its far-ultraviolet (FUV) instrument to map the diffuse FUV background—the faint ultraviolet light that is spread across the sky and does not originate from individual, easily identified sources. In the Milky Way, most of that glow is thought to come from starlight that has been scattered by interstellar dust. But even after accounting for the combined light of the galaxy’s hundreds of billions of stars, astronomers still found an excess of FUV radiation. Later work showed that this extra light emission likely originates within the Milky Way itself, but its distribution suggested it was not coming from just any random part of the galaxy.

“[AQNs] have a mass greater than a few grams and are sub-micrometer in size,” a group of researchers (led by Sekatchev) wrote in a study recently published in the Journal of Cosmology and Astroparticle Physics. “They would also help to explain the matter-antimatter asymmetry and the similarity between visible and dark components of the universe [by allowing researchers to calculate] the FUV electromagnetic signature in a [region] surrounding the Solar System, resulting from interaction between AQNs and baryons.”

This was further complicated by the fact that the brightness of the GALEX observations aligned with previous observations made by the Dynamics Explorer (also powered by NASA), which was much farther from Earth. This meant that the excess FUV radiation couldn’t be coming from terrestrial bodies or the Solar System. Even stranger was the finding that that this radiation was spread out evenly, unlike UV light emitted by stars, which isn’t usually so consistent. The excess also didn’t line up with the galactic longitudes of the Milky Way’s brightest UV-emitting stars, suggesting that it wasn’t simply unresolved starlight. Observations from the Alice UV spectrograph aboard New Horizons reinforced that conclusion: while roughly half of the FUV intensity measured could be traced to known UV sources, the rest remained unexplained.

The unusually smooth distribution of the FUV glow led Sekatchev to consider whether it could be produced by dark matter annihilation. After finding that several existing models didn’t match the GALEX data, he turned to an earlier speculative study that described a composite, electrically neutral dark antimatter object whose charged constituents could still interact with ordinary matter. If such an object exists, encounters with visible matter could trigger annihilation and produce some of the unexplained light. Axion quark nuggets are one object in a hypothetical class that fits that description.

Sekatchev and his team turned to computer simulations, figuring out how much FUV light would be emitted by AQNs if they made up the dark matter distribution that had already been determined for certain parts of the Milky Way. The results were a match for the GALEX and New Horizons findings on which they’d based their research. The team also found that ionizing photons produced in these matter-antimatter annihilations could help explain other long-standing astrophysical puzzles. “Recent observations from the James Webb Space Telescope reveal that early, faint galaxies are prolific producers of ionizing photons,” Sekatchev said. “Whether this can be sufficient to explain the JWST result without a significant change […] remains to be demonstrated.”

Dark matter may be invisible, but if Sekatchev’s team is right, it isn’t entirely silent. That phantom glow may turn out to be its calling card.