Something Deeply Weird Is Happening at the Core of Our Galaxy

There’s a new twist in the hunt for dark matter, the invisible substance believed to make up 85 percent of all the mass in the universe: it may actually be way lighter.

In a study published in the journal Physical Review Letters, an international team of researchers propose a new form of the hypothetical substance that’s lower in mass compared to other dark matter candidates, which could explain a mysterious phenomenon at the center of our Milky Way galaxy, in a region called the Central Molecular Zone (CMZ).

“At the center of our galaxy sit huge clouds of positively charged hydrogen, a mystery to scientists for decades because normally the gas is neutral,” said study co-lead author Shyam Balaji at King’s College London in a statement about the work. “So, what is supplying enough energy to knock the negatively charged electrons out of them?”

“The energy signatures radiating from this part of our galaxy suggest that there is a constant, roiling source of energy doing just that,” Balaji added, “and our data says it might come from a much lighter form of dark matter than current models consider.”

While scientists have extensive evidence that dark matter exists, determining what it is and where it resides remains one of the biggest questions in physics, with theories ranging from parallel universes to primordial black holes. But one of the original and still leading explanations for dark matter is that it comprises a type of nearly undetectable particles called Weakly Interacting Massive Particles, or WIMPs. 

As the moniker suggests, WIMPs don’t interact strongly with other particles, including light, but are massive enough that they would clump together in the way that astronomers have observed galaxies and other cosmic structures do — which dark matter, through its gravitational influence, is hypothesized to govern the shapes of from the shadows.

But perhaps there’s more than one form of it that’s less massive than WIMPs, as the researchers suggest. The thinking goes that in the extremely dense environment of the CMZ, these lighter dark matter particles would be constantly colliding and destroying each other upon impact, and subsequently releasing energy. This is a process known as annihilation, and the energy it liberates would then ionize nearby hydrogen gas.

Conversely, the researchers argue that WIMPs and other proposed dark matter particles like axions don’t undergo enough annihilation to ionize hydrogen to the extent observed in the CMZ. Nor can the phenomenon be explained by cosmic rays, powerful beams of energetic particles that zip throughout the universe at nearly the speed of light.

“The biggest problem this model helps solve is an excess of ionization in the CMZ,” Balaji told Space.com. “Cosmic rays, the usual culprits for ionizing gas, don’t seem to be strong enough to explain the high levels of ionization we observe.”

There’s still a lot of work to be done before the idea gains more steam. But if the theory holds true, Balaji says, we’d have an “entirely new way” to study dark matter rather than just its gravitational influence; now, we could observe the ionization it causes in gases.

“Dark matter remains one of the biggest mysteries in physics, and this work shows that we may have been overlooking its subtle chemical effects on the cosmos,” Balaji told Space.com.

More on space: Scientists Intrigued by Galactic Structure That’s 1.4 Billion Light-Years Wide