Dark matter near the center of our galaxy is “flattened,” not round as previously thought, new simulations reveal. The discovery could point to the origin of a mysterious high-energy glow that has intrigued astronomers for more than a decade, although more research is needed to rule out other theories.
“When the Fermi space telescope pointed toward the galactic center, it measured too many gamma rays,” Maurits Mihkel Mururesearcher at the Leibniz Institute for Astrophysics in Potsdam, Germany, and the University of Tartu in Estonia, told Live Science by email. “Different theories compete to explain what could produce this excess, but no one has a definitive answer yet.”
From the beginning, scientists propose that the glow could come from dark matter the particles collide and annihilate each other. However, the flattened shape of the signal did not match the spherical halos assumed in most dark matter models. This divergence has led many scientists to favor a alternative explanation involving millisecond pulsars – ancient, rapidly rotating neutron stars that emit gamma rays.
Now, a study published October 16 in the journal Physical Examination Letters and led by Muru challenges the long-held hypothesis regarding the shape of dark matter. Using advanced simulations of Milky WayMuru and his colleagues discovered that dark matter near the galactic center is not perfectly round, but flattened, just like the observed gamma signal.
A persistent cosmic puzzle
Gamma rays are the most energetic form of light. They are often produced in the most extreme environments in the universe, such as violent stellar explosions and matter swirling around black holes. Yet even after accounting for known sources, astronomers still discovered an unexplained glow coming from the core of the Milky Way.
One proposed explanation is that the radiation comes from dark matter, the invisible substance that makes up most of the mass in the universe. Some models suggest that dark matter particles can occasionally crash into each other, converting some of their mass into gamma-ray bursts.
“Since there are no direct measurements of dark matter, we don’t know much about it,” Muru said. “One theory is that dark matter particles can interact with each other and annihilate each other. When two particles collide, they release energy in the form of high-energy radiation.”
But this theory fell out of favor when the flattened, disk-like shape of gamma rays failed to match the hypothetical shape of dark matter halos – thought to be spherical.
Rethinking the shape of dark matter
Muru and his colleagues set out to revisit the basic assumption that dark matter in the inner galaxy must be spherical. Using high-resolution computer simulations known as the HESTIA suite, which recreates Milky Way-like galaxies in a realistic cosmic environment, the team studied the behavior of dark matter near the galactic center.
They found that past mergers and gravitational interactions can distort the distribution of dark matter, flattening it into an oval or box-like shape, much like the bulge of stars seen in the middle of our galaxy.
“Our most important result was to show that the reason the dark matter interpretation was unfavorable came from a simple assumption,” Muru said. “We discovered that dark matter near the center is not spherical, but flattened. This brings us closer to revealing what dark matter actually is, using clues from the heart of our galaxy.”
This revised picture means that the gamma-ray pattern expected from dark matter annihilation could naturally look a lot like what astronomers observe. In other words, the dark matter explanation might have been underestimated simply because scientists were using the wrong form.
What comes next
Although the new findings strengthen the case for dark matter as the origin of the gamma signal, they do not close the debate. To distinguish dark matter from pulsars, astronomers need more precise observations.
“A clear indication of the stellar explanation would be the discovery of enough pulsars to explain the gamma-ray glow,” Muru said. “New telescopes with higher resolution are already under construction, which could help resolve this question.”
If upcoming instruments, such as the Square Kilometer Array (SKA) and the Cherenkov Telescope Array (CTA), reveal many tiny, point sources at the galactic center, this would favor the pulsar explanation. If, on the contrary, the radiation remains regular and diffuse, the dark matter scenario would gain popularity.
“A compelling proof of dark matter would be a signal that precisely matches theoretical predictions,” Muru noted, adding that such confirmation would require both improved modeling and better telescopes. “Even before the next generation of observations, our models and predictions are steadily improving. A 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 rays has lasted for more than 10 years, with each new study adding a piece to the puzzle. Whether the glow comes from dark matter, pulsars, or something completely unexpected, Muru’s results highlight how the structure of the galaxy itself may hold vital clues. By reshaping our understanding of the Milky Way’s dark core, scientists are getting closer to answering one of the most profound questions in modern astrophysics: what dark matter actually is.
