Scientists think the mysterious glow in our galaxy could be from dark matter. What that means

The Milky Way, seen here in the Uruguayan sky, has a mysterious glow at its center, which could be related to a hidden form of matter that physicists call dark matter. (Mariana Suarez/AFP/Getty Images via CNN Newsource)

At the centre of our galaxy, there’s a mysterious, diffuse glow given off by gamma rays — powerful radiation usually emitted by high-energy objects such as rapidly rotating or exploding stars.

NASA’s Fermi Gamma-ray Space Telescope detected the glow shortly after launching in 2008, and the light has puzzled scientists ever since, prompting speculation on its cause.

Some astronomers believe the source of the glow to be pulsars — the spinning leftovers of exploded stars — while others point to colliding particles of dark matter, an elusive and invisible form of matter that is believed to be five times more abundant than regular matter.

Many studies have previously found support for both ideas, but there seemed to be a problem with the dark matter theory: The gamma ray glow appeared to match the shape of the galactic bulge — a crowded, bulbous region at the centre of the Milky Way that’s mostly made up of old stars, including pulsars. This observation seemed to support the pulsar theory, with experts theorizing that the glow would have taken a more spherical form if its source were dark matter. However, astronomers haven’t been able to observe enough of the pulsars that would be producing the gamma rays to make a conclusive assessment.

Now, new simulations made using supercomputers show for the first time that dark matter collisions could also have created the bulge-shaped glow, adding weight to the dark matter theory.

“We’re in the situation where we have two theories, one posturing dark matter and claiming that it could explain the data we see, another one old stars,” said Joseph Silk, a professor of physics and astronomy at the Johns Hopkins University and coauthor of a study detailing the new findings, published Thursday in the journal Physical Review Letters.

“There’s a 50% chance that it might be dark matter at this point, as opposed to the slightly more mundane explanation of old stars, in my opinion.”

Evidence of dark matter would make for a groundbreaking discovery. Swiss astronomer Fritz Zwicky first theorized dark matter’s existence in the 1930s, and American astronomers Vera Rubin and W. Kent Ford confirmed it in the 1970s. They noticed that stars orbiting at the edge of spiral galaxies were moving too quickly to be held together by visible matter and gravity alone, and postulated that there was a large, unseen quantity of matter preventing them from flying apart. Despite decades of effort, scientists have never observed the mysterious substance directly, hence its name.

“There’s no question that the nature of dark matter is one of the outstanding major problems in physics,” Silk said. “It’s something that’s everywhere — near us, far from us, and we just don’t know what it is.”

Hunting for WIMPs

There are many hypotheses on what dark matter could be, including remnants of primordial black holes or an undiscovered type of particle. Much of the effort to find dark matter has centred on the latter idea, leading to the construction of detectors such as the LZ Dark Matter Experiment in South Dakota.

The instrument is designed to spot one of the leading dark matter candidates, hypothetical particles called WIMPs — Weakly Interacting Massive Particles — which don’t absorb light and can pass through regular matter almost seamlessly. Scientists believe that when two WIMPs meet, they annihilate each other and produce gamma rays, which would make them a plausible source of the glow.

Silk’s study used supercomputers to create a map of where dark matter should be in the Milky Way, taking into account how the galaxy originally formed.

“The problem was that all of the models over the past 20 years of the dark matter in our galaxy assume it’s basically like a spherical ball. There is no shape to it, because that was the simplest model,” Silk said.

“Our contribution was, for the first time, to make a real computer simulation of the dark matter distribution. And lo and behold, we found the central part of the dark matter, where the gamma rays would be being emitted, to be, in fact, squashed — more like egg-shaped.” This squashed shape is a close match to the Fermi telescope data, Silk explained.

Luckily, confirmation of the link between dark matter and the glow may not be too far in the offing. A new instrument, the Cherenkov Telescope Array Observatory, or CTAO, is under construction at two sites — one in Chile and another in Spain — and will start to return data as soon as 2027. CTAO will detect gamma rays at a much higher resolution than Fermi, Silk said, making it possible to tell if the gamma rays at the centre of the Milky Way are the product of dark matter collisions.

That finding would be a breakthrough in the search for the elusive substance, he added, as well as offer proof that at least some dark matter is made of WIMPs. If, on the contrary, CTAO doesn’t link the glow to dark matter, scientists would be back to square one in the search, with all options still on the table.

A fundamental secret

The study helps reopen the possibility that dark matter could explain the glow at our galactic centre, although it doesn’t give new positive evidence in favor of dark matter, said Tracy Slatyer, a professor of physics at the Massachusetts Institute of Technology who was not involved with the study. However, she is not convinced that there is a definitive match between the shape of the dark matter distribution and the stellar bulge. “I thought the dark matter hypothesis was still reasonable even before this study,” she added.

This work is further support for the international effort to keep pushing in the hunt for WIMPs, according to Chamkaur Ghag, a professor of physics and astronomy at University College London, who also didn’t participate in Silk’s research. “They remain a most elegant solution to the long-standing dark matter problem,” Ghag added via email, noting that with even more detectors for WIMPs under development, seeing signals of these particles annihilating in space would mean settling the near-century old puzzle of dark matter.

Nico Cappelluti, an associate professor in the department of physics at the University of Miami, said that the Fermi telescope has been a game changer for NASA, and this paper shows that dark matter is still very much in the race to explain the strange glow at the centre of our galaxy. “That mystery is alive, and it’s the kind that keeps scientists like me awake at night,” said Cappelluti, who didn’t take part in the study.

Figuring out what dark matter is has been the scientific quest of our century, he added, noting that “WIMPs, these hypothetical particles, have been our prime suspects for years.” The fact that experiments on Earth haven’t caught them yet is frustrating, he said.

“But Fermi gives us a reason to keep believing. This paper reminds us not to cross WIMPs off the list just yet — they might still be lighting up the centre of our galaxy,” Cappelluti said. “And if that’s true, we’re closer than ever to uncovering a fundamental secret of the universe.”