Dark matter is a mysterious and currently hypothetical substance that outweighs regular matter five to one. Theory and observations strongly suggest that dark matter is out there but so far we have failed to find it.
A piece of previously discarded evidence is the galactic center excess, or GCE, an unexplained emission of powerful gamma rays from the core of the Milky Way. Studies have shown that the most likely explanation is that the emission is caused by pulsars and not by colliding dark matter. But authors from one of those studies now think that dark matter can’t be completely ruled out as the cause.
In 2015, Professor Tracy Slatyer was part of a collaboration between MIT and Princeton University that developed a computer model. This showed that the observations from the Fermi Gamma-Ray Space Telescope were best explained by pulsars, which are extremely dense neutron stars spinning at incredible speeds.
In a paper published in Physical Review Letters, Slatyer and lead author Dr Rebecca Leane have discovered that their model is not error-proof. They introduced a fake “dark matter signal” into the data but the model failed to pick up on the signal and still preferred the scenario where pulsars were responsible.
This doesn’t mean that the real signal is dark matter but their approach suggests that the possibility that the emission is at least partially due to dark matter cannot be discounted.
"It's exciting in that we thought we had eliminated the possibility that this is dark matter," Slatyer said in a statement. "But now there's a loophole, a systematic error in the claim we made. It reopens the door for the signal to be coming from dark matter."
The approach first used in 2015 is very inventive in its simplicity. The GCE is found in a spherical region of about 5,000 light-years around the galactic center point. If the signal was produced by pulsars, it should appear grainy as pulsars are not distributed everywhere; they are found in specific places with gaps all around. Meanwhile, if the cause was dark matter, the emission should be smooth across the whole GCE as dark matter does not clump together.
"My hope was that this would be just the first of many studies of the galactic center region using similar techniques," Slatyer explained. "But by 2018, the main cross-checks of the method were still the ones we'd done in 2015, which made me pretty nervous that we might have missed something."
The original study found that the emission looks 100 percent grainy. But when the fake dark matter emission was brought into the equation, the model failed to spot it. And for the researchers, this means that dark matter cannot be discounted anymore.
"If it's really dark matter, this would be the first evidence of dark matter interacting with visible matter through forces other than gravity," Leane said. "The nature of dark matter is one of the biggest open questions in physics at the moment. Identifying this signal as dark matter may allow us to finally expose the fundamental identity of dark matter. No matter what the excess turns out to be, we will learn something new about the universe."
Leane and Slatyer are now working to improve the model and uncover exactly what’s going on. They hope to tackle the issue and finally get to the bottom of this enduring mystery.