“At the centre of our galaxy sit huge clouds of positively charged hydrogen, a mystery to scientists for decades because normally the gas should stay neutral. So, what is supplying enough energy to knock the negatively charged electrons out of them?” said Dr Shyam Balaji of King’s College London in a statement. 

Physicists originally attributed the ionization to cosmic rays, but other measurements suggest there aren’t enough of them to do the job. Balaji and co-authors went looking for an alternative. They also noted the frequent detection of 511 electron volt (eV) gamma rays coming from the galactic bulge. This is the radiation produced when electrons and positron pairs decay, turning the mass of both into energy, but the source of these pairs is unknown.

Balaji and co-authors noted that electron-positron pairs can ionize hydrogen, so a suitable source of electron and positron pairs might explain both observations. Collisions between certain subatomic particles can produce matter/antimatter pairs, so one possible explanation is that particles are slamming into each other in the crowded inner reaches of the galaxy and producing electron-positron pairs. Some of these go on to ionize the hydrogen, while others (or perhaps the same ones at a later time) run into each other, creating 511 eV light in their annihilation.

The authors think we can rule out all known particles as being the cause. If the responsible particles have mass, they would account for some – perhaps all – of the dark matter that scientists have been chasing for decades.

Nice as it would be to find a single solution to three problems, the team needed to show their idea was plausible if they wanted people to go looking for evidence. They considered the characteristics required of a particle to be indirectly responsible for appropriate pair production. According to a newly published paper, there’s an overlap where a particle with a narrow range of characteristics could do both. They say these particles would be a sort of light dark matter, if they are real.

If you’re wondering how can matter be both light and dark at the same time, it’s true that physicists have discovered weirder things, but in this case, the problem is with the English language, not reality. Dark matter gets its name because it does not interact with the electromagnetic force, and therefore produces no light. The light dark matter Balaji and co-authors are interested in would be similarly dark, but is made up of low-mass particles, “light” here being opposite to heavy, not dark.

The particles in question would have about a thousandth of the mass of WIMPS, one of the first proposed dark matter candidates. There would need to be an awful lot of them to explain the vast quantities of dark matter needed to make sense of the way galaxies rotate and evolve.

However, Balaji told IFLScience that this is plausible. “These dark matter particles should be widespread throughout the universe, just like other dark matter candidates,” Balaji said. “However, their effects are only noticeable in regions where dark matter is most concentrated, such as the centre of the Milky Way. Since the annihilation rate depends on density squared, dark matter particles are much more likely to interact in areas of high density, making the central molecular zone (CMZ) a natural place to look for their influence.”

Such light particles might also be harder to detect using existing methods than longer-standing candidates like WIMPS and axions, explaining the ongoing frustrations of the dark matter search.

Balaji told IFLScience that even without finding such a particle on Earth and confirming its properties, there are at least three ways to test the idea.

“More detailed ionisation maps. If ionisation rates in the CMZ match the expected distribution of dark matter, that would strengthen the case,” he said. Another way could be to look for secondary emissions, explained Balaji. “If these particles exist, they should also leave subtle traces in the form of weak gamma-ray or X-ray signals from secondary processes.” 

Finally, Balaji added, “Upcoming telescopes. NASA’s COSI space telescope, due to launch in 2027, will be sensitive to MeV-scale astrophysical processes, which could provide evidence for or against this hypothesis.”

Positrons can ionize hydrogen by annihilating the electron, but in such cases, the excess of electrons could restore neutrality. However, Balaji told IFLScience that the likely process is more complex. The pairs have a lot of energy when created, enough to knock electrons off atoms through collisions, with each pair ionizing several hydrogen atoms before it loses enough energy to stop. Think a toddler bumping tables and shaking items on top off, not a bomb destroying them.

The study is published in Physical Review Letters.