Dark matter near the center of our galaxy is “flattened” rather than round as previously thought, new simulations show. The discovery may point to the origin of a mysterious high-energy glow that has puzzled astronomers for more than a decade, although more research is needed to rule out other theories.
“When the Fermi Space Telescope went to the center of the galaxy, it measured too many gamma rays,” Mooritz Mihkel Mururesearcher at the Leibniz Institute for Astrophysics Potsdam in Germany and the University of Tartu in Estonia, told Live Science via email. “Various theories are vying to explain what could be causing this excess, but there is no clear answer yet.”
In the beginning, scientists proposed that the glow may come from dark matter particles collide and destroy each other. However, the flattened signal shape did not correspond to the spherical halos assumed by most dark matter models. This discrepancy has led many scientists to prefer alternative explanation involving millisecond pulsars—ancient, rapidly spinning neutron stars that emit gamma rays.
Now the study, published Oct. 16 in the journal Physical Review Letters and, under Muru's leadership, challenges long-held assumptions about the shape of dark matter. Use of advanced simulation models Milky WayMuru and his colleagues found that the dark matter near the center of the galaxy is not perfectly round, but flattened – just like the observed gamma-ray signal.
An ongoing cosmic mystery
Gamma rays are the most energetic form of light. They are often produced in the universe's most extreme conditions, such as violent stellar explosions and matter orbiting black holes. However, even after accounting for known sources, astronomers have consistently detected an inexplicable glow emanating from Milky Way'check.
One proposed explanation is that the radiation comes from dark matter, the invisible substance that makes up most of the mass of the universe. Some models suggest that dark matter particles may occasionally collide with each other, turning some of their mass into bursts of gamma rays.
“Because there are no direct measurements of dark matter, we know little about it,” Mourou said. “One theory is that dark matter particles can interact with each other and annihilate. When two particles collide, they release energy in the form of high-energy radiation.”
But this theory fell out of favor when the flattened disc-shaped shape of gamma rays did not match the assumed shape of dark matter halos, which are thought to be spherical.
Rethinking the Shape of Dark Matter
Muru and his colleagues set out to reconsider the basic assumption that dark matter in the inner galaxy should be spherical. Using high-resolution computer simulations known as the HESTIA package, which recreates galaxies like the Milky Way in realistic space environments, the team studied how dark matter behaves near the galactic center.
They found that past mergers and gravitational interactions can distort the distribution of dark matter, giving it an oval or box-like shape—much like the bulging stars seen at the center of our galaxy.
“Our most important result showed that the reason the dark matter interpretation was not accepted came from a simple assumption,” Muru said. “We found that dark matter near the center is not spherical, but flattened. This brings us one step closer to uncovering what dark matter really is, using clues coming from the heart of our galaxy.”
This revised picture means that the gamma-ray pattern expected from dark matter annihilation may naturally look very similar to what astronomers observe. In other words, the dark matter explanation may have been underestimated simply because scientists used the wrong shape.
What happens next
While the new discoveries bolster the case for dark matter as the source of the gamma-ray signal, they do not end the debate. To distinguish dark matter from pulsars, astronomers need more precise observations.
“A clear indication of a stellar explanation would be the discovery of enough pulsars to explain the gamma-ray glow,” Muru said. “New telescopes with higher resolution are already being built that could help resolve this issue.”
If future instruments such as the Square Kilometer Array (SKA) and the Cherenkov Telescope (CTA) detect many tiny point sources at the center of the galaxy, this will help explain pulsars. If instead the radiation remained smooth and diffuse, the dark matter scenario would gain support.
The “smoking gun” for dark matter would be a signal that exactly matches theoretical predictions, Muru said, adding that such confirmation would require both improved modeling and more advanced telescopes. “Even before the next generation of observations, our models and forecasts are steadily improving. One future prospect is to find other places to test our theories, such as the central regions of nearby dwarf galaxies.”
The mystery of excess gamma radiation has persisted for more than 10 years, and each new study adds a piece of the puzzle. Whether the glow comes from dark matter, pulsars, or something completely unexpected, Muru's results show that the structure of a galaxy itself may hold vital clues. By changing our understanding of the Milky Way's dark core, scientists are getting closer to answering one of the deepest questions in modern astrophysics—what dark matter really is.
