Invisible but omnipresent, dark matter constitutes approximately 27% of the known universe. Yet despite decades of research, no particle has ever been directly detected. It does not shine, does not absorb light, and escapes all observation instruments. Astrophysicists can only infer its existence through its gravitational influence on ordinary matter – that which constitutes the stars, the planets, and ourselves.
But an unexpected clue could betray its presence: a ghostly glow in the ultraviolet. Surveys carried out in our galaxy have revealed a diffuse luminosity that no one can fully explain. According to a recent hypothesis relayed by Popular Mechanics, this faint glow could well come from a very particular form of dark matter.
Among the many avenues considered to explain the nature of dark matter, some theories propose that it is composed of ultradense objects called Axion Quark Nuggets (AQN), “axion quark nuggets”. These tiny entities would associate particles called quarks with axions, the existence of which remains hypothetical but could resolve several cosmic enigmas.
AQNs would behave similarly to “cold dark matter,” believed to play a major role in galaxy formation. However, their singularity would be due to a particularity: when they interact with ordinary matter, they could emit light in the form of electromagnetic radiation. And if some of these “nuggets” were composed of antimatter, these collisions would result in spectacular annihilations, transforming the mass into luminous bursts.
Excessive ultraviolet light
This is the scenario that Michael Sekatchev, an astrophysicist at the University of California, Berkeley, wanted to test. By studying data collected around ten years ago by NASA’s GALEX satellite, he looked at this diffuse ultraviolet background. This tenuous light bathes our universe without any obvious source having been identified. Part of this luminosity comes from star radiation, scattered by interstellar dust, but once this contribution is subtracted, an excess remains, which researchers cannot explain.
Other observations, notably those of the Dynamics Explorer probe and the Alice spectrograph on board New Horizons, have confirmed this anomaly: the ultraviolet glow is distributed very uniformly and does not correspond to the distribution of the brightest stars in the Milky Way. In other words, it does not come from simple stellar noise.
A glow from dark matter?
This homogeneity convinced Michael Sekatchev to test a daring hypothesis: what if this light came from a process of annihilation between matter and dark matter? In previous theoretical work, certain researchers had described objects composed of dark matter but capable, under certain conditions, of interacting slightly with visible matter. The AQNs matched this profile.
Using computer simulations, the Berkeley team calculated how much ultraviolet light could be produced if NWAs made up the dark matter in the regions of the Milky Way already studied. The results agreed with the GALEX and New Horizons measurements: the model precisely reproduced the observed brightness.
Better yet, the ionizing photons released during these annihilations could shed light on other cosmic mysteries. Observations from the James Webb Space Telescope have shown that the universe’s earliest galaxies produced immense amounts of light capable of ionizing surrounding gas. If AQNs exist, their interactions could have contributed to this phenomenon, offering a key explanation without major disruption to the current cosmological model.
The researchers estimate that these “nuggets” would have a mass of only a few grams and a size less than a micrometer, but their density would be so extreme that they could have a measurable effect on a large scale. According to their study published in the Journal of Cosmology and Astroparticle Physics, the interaction between AQN and baryons (the particles constituting ordinary matter) would make it possible to reproduce the electromagnetic signature observed around the solar system.
This hypothesis does not yet prove the existence of AQN, but it opens a new avenue in the hunt for dark matter. If such objects do indeed exist, their light trace could become a valuable tool for better understanding the composition of the universe.
