At the center of our galaxy is a mysterious, diffuse glow emitted by gamma rays, the powerful radiation normally emitted by high-energy objects such as rapidly spinning or exploding stars.
NASA’s Fermi Gamma-ray Space Telescope detected the glow shortly after launch in 2008, and the light has baffled scientists and fueled speculation about its cause ever since.
Some astronomers believe that the source of the glow is pulsars, the spinning remnants of exploded stars, while others point to colliding particles of dark matter, an elusive and invisible form of matter believed to be five times more abundant than normal matter.
Many studies had previously found both points, but there seemed to be a problem with the dark matter theory: The gamma-ray glow seemed to match the shape of the galactic bulge, a crowded, bulbous region at the center of the Milky Way made up mostly of old stars, including pulsars. The observation seemed to support the pulsar theory, and experts said the glow would have taken on a more spherical shape if dark matter had been its source. But astronomers haven’t been able to observe enough pulsars that emit gamma rays to make a definitive estimate.
Now, new models built using supercomputers show for the first time that dark matter collisions could also have produced a bulge-shaped glow, boosting the theory of dark matter.
“We’re in a situation where we have two theories, one that says it can explain the data we see, and the other that explains the old stars,” said Joseph Silk, a professor of physics and astronomy at Johns Hopkins University and co-author of the study detailing the new findings.
“There’s a 50% chance that it could be dark matter at this point, rather than what I think is a slightly more mundane explanation for old stars.”
The gamma-ray glow can be clearly seen in this image from data from NASA’s Fermi telescope along the center of the map, which marks the central plane of our Milky Way galaxy. – NASA/DOE/Fermi LAT collaboration
Evidence of dark matter would be a groundbreaking discovery. Swiss astronomer Fritz Zwicky first theorized the existence of dark matter in the 1940s, and American astronomers Vera Rubin and W. Kent Ford confirmed it in the 1970s. They found that stars orbiting at the edges of spiral galaxies are moving too fast to be held back by visible matter and gravity alone, and suggested that there is a large, invisible amount of matter preventing them from flying. Despite decades of effort, scientists have never directly observed the mysterious substance, hence its name.
Vera Rubin discovered in the 1970s that most of the universe is made up of “dark matter”. – The Washington Times/Shutterstock.
“There is no doubt that the nature of dark matter is one of the most important problems in physics,” said Silk. “It’s something that’s everywhere – around us, far away from us, and we just don’t know what it is.”
WIMP Hunt
There are many hypotheses for 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 focused on the latter idea, leading to detectors like the LZ Dark Matter Experiment in South Dakota.
The instrument is designed to detect one of the leading candidates for dark matter, hypothetical particles called WIMPs (weakly interacting massive particles), which do not absorb light and can pass through ordinary matter almost seamlessly. Scientists believe that when two WIMPs meet, they annihilate each other and produce gamma rays, which would make them a reliable source of glow.
Silko’s study used supercomputers to create a map of where dark matter should be in a part of the Milky Way, based on how the galaxy originally formed.
“The problem has been that for the last 20 years, all the dark matter models in our galaxy have assumed that it’s basically like a spherical ball. It doesn’t have any shape because that was the simplest model,” Silk said.
“Our contribution was, for the first time, a real computer simulation of the distribution of dark matter. And lo and behold, we found that the central part of the dark matter, where the gamma rays would radiate, is actually compressed—more like an egg shape.” This compressed form is very similar to data from the Fermi telescope, Silk explained.
NASA’s Fermi Gamma-ray Space Telescope, shown here, scans the entire sky every three hours as it orbits Earth. – NASA Goddard Space Flight Center / Chris Smith (USRA / GESTAR)
Fortunately, the connection between dark matter and luminescence may not be too far off. The new instrument, the Cherenkov Telescope Array Observatory, or CTAO, is under construction at two sites — one in Chile and the other in Spain — and will start providing data as early as 2027. CTAO will detect gamma-rays at a much higher resolution than Fermi, Silk said, so it will be possible to determine whether the gamma-ray product of dark matter is at the center of the Way collision.
Such a discovery would be a breakthrough in the search for elusive matter, he added, and would also show that at least some dark matter is made up of WIMPs. By contrast, CTAO does not link the glow to dark matter, and scientists would be back to square one in their search, with all possibilities still under consideration.
The main secret
The study helps reopen the possibility that dark matter could explain the glow at the center of our galaxy, although it does not provide new positive evidence for dark matter, said Tracy Slatyer, a professor of physics at the Massachusetts Institute of Technology who was not involved in the study. However, she is not convinced that there is a definitive correspondence between the shape of the dark matter distribution and the stellar bulge. “I thought the dark matter hypothesis was still valid even before this study,” she added.
According to Chamkaur Ghag, professor of physics and astronomy at University College London, who was also not involved in Silk’s research, the work is another boost to international efforts to continue the search for WIMPs. “They remain the most elegant solution to the long-standing dark matter problem,” Ghag added in an email. via email, noting that even more WIMP detectors are being built, the annihilation signals of these particles in space would solve the nearly century-old dark matter puzzle.
The Central LZ Dark Matter Experiment Detector, located at the Sanford Underground Research Facility in South Dakota, is seen here before it was placed underground. – Matthew Kapust / Sanford Underground Research Facility / Lawrence Livermore National Laboratory
Nico Cappelluti, an assistant professor in the Department of Physics at the University of Miami, said the Fermi telescope was a game-changer for NASA, and the paper shows that dark matter is still struggling to explain the strange glow at the center of our galaxy. “That mystery lives on, and that’s what keeps scientists like me awake at night,” said Cappelluti, who was not involved 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 picked them up yet is frustrating,” he said.
“However, Fermi gives us a reason to keep believing. This paper is a reminder that WIMPs can’t be written off the list just yet – they can still illuminate our galactic center,” said Cappelluti. “And if that’s true, we’re closer than ever to unlocking the ultimate mystery of the universe.”
Sign up for CNN’s Wonder Theory science newsletter. Explore the universe with news about exciting discoveries, scientific advances and more.
For more CNN news and newsletters, create an account at CNN.com