Dark matter ‘nuggets’ could explain the Milky Way’s mysterious glow

A puzzling ultraviolet light seen across the Milky Way could come from the destruction of nuggets of dark matter, the mysterious stuff that makes up around a quarter of the matter and energy in the universe.

The light could originate with a type of dark matter called axion quark nuggets, researchers suggest in a paper submitted to arXiv.org. This kind of dark matter — if it exists — would come in both matter and antimatter flavors, providing it a way to make an ultraviolet glow that other dark matter particles can’t manage.

First theorized in 2003, axion quark nuggets are made of bundles of ordinary fundamental particles and a theoretical variety known as axions, which are very light and uncharged. These nuggets are incredibly dense, packing the mass of a golf ball into a clump just a micrometer across — smaller than the width of a human hair.

Unlike some other forms of dark matter, the researchers say, the axions allow some of these dark nuggets to be made of antimatter, a twin class of particles that are identical to ordinary matter but with the opposite electric charge. When a particle of matter meets a corresponding particle of antimatter, the pair mutually destroy each other and convert all their energy into light. That could be happening with dark matter nuggets, too.

“If you have regular matter colliding with these antinuggets, they can annihilate and radiate away some energy,” says study coauthor Michael Sekatchev, an astrophysicist now at the University of California, Berkeley. “And that’s the glow.”

The glow seen across the Milky Way is a slight excess of far-ultraviolet light, more than what could be accounted for by all the known stars, gas, and dust within the galaxy. Astronomers first noticed the surplus in the 2010s, and over the years researchers determined that the light — which has the shortest wavelengths in the ultraviolet part of the electromagnetic spectrum — must be coming from somewhere beyond the solar system. But they couldn’t satisfactorily explain what was emitting the light.

Leading explanations have included different theoretical types of dark matter made of exotic particles. But none of these hypothetical particles have stacked up with the evidence.

To check if a collision of antinuggets and ordinary matter could produce the mysterious far-ultraviolet light, Sekatchev and colleagues studied existing computer simulations of the Milky Way. Using the distribution of gas and dark matter in test regions of the simulated galaxy, the team calculated how much far-ultraviolet light would be emitted by axion quark nuggets based on their theoretical properties. The results matched the amounts of extra far-ultraviolet light seen by spacecraft like the Galaxy Evolution Explorer and New Horizons.

The results are preliminary, notes theoretical astrophysicist James Overduin of Towson University in Maryland. “But I believe the authors have shown convincingly that axion quark nuggets can explain an otherwise inexplicable part of the diffuse galactic background,” he says. “I am not aware of any other dark matter candidate for which that can be said.”

More tests and observations are needed to see if the new theory holds up. But if the ultraviolet light is indeed caused by dark matter nuggets, the finding would help scientists better understand dark matter and other mysteries in physics and astronomy, such as why there’s so much more matter than antimatter in the universe.