For more than a decade, astronomers have been puzzled by an unusual glow of high-energy gamma-rays emanating from the center of the Milky Way. This mysterious light, first detected by NASA’s Fermi Gamma-ray Space Telescope, exceeds what scientists expected from known sources like supernovae or matter swirling around black holes. The origin of this glow has long been debated, with theories ranging from fast-spinning neutron stars to the elusive dark matter. Now, new simulations suggest that our assumptions about the shape of dark matter itself may need an update — and that could change the way we interpret this cosmic signal.
Moorits Mihkel Muru, a researcher at the Leibniz Institute for Astrophysics Potsdam in Germany and the University of Tartu in Estonia, has been at the forefront of this investigation. Speaking to Live Science, he explained that the excess gamma-rays observed near the galactic center have puzzled astronomers precisely because they did not conform to the shapes predicted by standard dark matter models. “Different theories compete to explain what could be producing that excess, but nobody has the definitive answer yet,” Muru said.
Early in the debate, one of the leading explanations suggested that the gamma-ray glow might result from dark matter particles colliding and annihilating each other. Dark matter, which makes up roughly 85% of the universe’s mass, is invisible and does not interact with light, making it extremely difficult to study. Some theoretical models predict that when dark matter particles collide, they convert part of their mass into energy, producing gamma-rays detectable by instruments like Fermi.
However, there was a problem: the gamma-ray signal did not appear spherical. For years, many dark matter models assumed that dark matter halos — the structures surrounding galaxies — would have roughly spherical shapes, particularly near the center. Observations, however, suggested a more flattened, disk-like glow. This discrepancy prompted some scientists to favor alternative explanations, most notably millisecond pulsars — old, fast-spinning neutron stars capable of emitting gamma-rays.
Revisiting Dark Matter Assumptions
The new study, published on October 16 in Physical Review Letters, challenges the long-held assumption that dark matter near the galactic center must be spherical. Using high-resolution simulations from the HESTIA project — a computational suite designed to replicate Milky Way-like galaxies in realistic cosmic environments — Muru and his team investigated how dark matter behaves under the influence of gravitational interactions and galaxy formation processes.
The results were striking. The simulations revealed that dark matter near the galaxy’s core is not round but flattened, resembling the bulge of stars seen at the center of the Milky Way. Past mergers and gravitational disturbances appear capable of distorting the dark matter halo into an elongated or box-like shape. “Our most important result was showing that a reason why the dark matter interpretation was disfavored came from a simple assumption,” Muru said. “We found that dark matter near the center is not spherical — it’s flattened. This brings us a step closer to revealing what dark matter really is, using clues coming from the heart of our galaxy.”
In practical terms, this reshaped understanding suggests that the gamma-ray glow, previously thought to be inconsistent with dark matter models, might actually align with the expected emissions from a flattened dark matter distribution. In other words, some of the skepticism around dark matter as the source of the excess may have stemmed from an overly simplistic geometric assumption.
Gamma-rays: A Persistent Cosmic Puzzle
Gamma-rays are the most energetic form of light, often produced in the universe’s most extreme environments. Supernova explosions, matter accreting onto black holes, and collisions between high-energy cosmic rays all generate gamma-rays. Yet the excess radiation observed near the Milky Way’s center could not be fully explained by these sources alone, leaving scientists searching for alternative explanations.
One hypothesis is that dark matter annihilation produces these high-energy emissions. “As there are no direct measurements of dark matter, we don’t know a lot about it,” Muru explained. “One theory is that dark matter particles can interact with each other and annihilate. When two particles collide, they release energy as high-energy radiation.”
For years, the flattened appearance of the gamma-ray glow cast doubt on this idea. Scientists assumed that dark matter halos would be round, making a disk-shaped gamma-ray signal incompatible with the theory. Now, the new simulations indicate that this assumption may have been flawed.
How the New Simulations Work
The HESTIA simulations employed by Muru’s team model galaxies with unprecedented detail, including the gravitational influence of stars, gas, and dark matter. By recreating a Milky Way-like environment, the researchers were able to observe how dark matter distribution evolves over billions of years.
The key takeaway: gravitational interactions, including past mergers with smaller galaxies, can flatten dark matter halos in a manner similar to the visible stellar bulge. This insight bridges the gap between the expected theoretical gamma-ray signal from dark matter annihilation and the observed disk-like shape in Fermi data.
“Our models show that the dark matter halo can be significantly distorted near the center, which helps reconcile theory with observation,” Muru said. “This gives new weight to the idea that dark matter might actually be contributing to the gamma-ray excess.”
Pulsars Remain a Competing Theory
Despite these encouraging results for dark matter, the alternative explanation — millisecond pulsars — remains plausible. These ancient neutron stars spin hundreds of times per second, emitting beams of gamma-rays detectable from Earth. If enough pulsars exist near the galactic center, their collective emissions could account for the observed glow.
Future observations will be crucial to distinguish between these scenarios. Instruments with higher resolution, like the upcoming Square Kilometre Array (SKA) and Cherenkov Telescope Array (CTA), could identify individual point sources. If these telescopes detect numerous compact sources, the pulsar hypothesis would gain support. Conversely, if the gamma-ray emission remains smooth and diffuse, the case for dark matter would strengthen.
“A clear indication for the stellar explanation would be the discovery of enough pulsars to account for the gamma-ray glow,” Muru said. “New telescopes with higher resolution are already being built, which could help settle this question.”
Implications for Dark Matter Research
Understanding the shape of dark matter halos has broader implications for cosmology. Accurate models of dark matter distribution help scientists test fundamental theories about the universe’s formation and evolution. They also provide constraints on the particle properties of dark matter, such as mass and interaction cross-sections.
If dark matter indeed produces the Milky Way’s gamma-ray excess, it would mark one of the first indirect detections of this mysterious substance. However, researchers caution that confirmation requires both refined theoretical modeling and next-generation observations.
“A ‘smoking gun’ for dark matter would be a signal that matches theoretical predictions precisely,” Muru noted. “Even before the next generation of observations, our models and predictions are steadily improving. One future outlook is to find other places to test our theories, such as the central regions of nearby dwarf galaxies.”
The Decade-Long Mystery
The gamma-ray excess at the center of our galaxy has persisted as an astronomical puzzle for more than ten years. Each new study adds pieces to the puzzle, whether by narrowing down possible sources, refining models, or challenging assumptions about the Milky Way’s structure.
Muru’s study demonstrates how revisiting foundational assumptions — in this case, the shape of dark matter halos — can change the interpretation of longstanding mysteries. By accounting for a flattened dark matter distribution, researchers have opened the door to re-evaluating the gamma-ray glow as a potential signal of dark matter annihilation.
Looking Ahead
The upcoming generation of telescopes promises sharper, more detailed views of the galactic center. As SKA, CTA, and other instruments come online, astronomers hope to identify the true sources of high-energy radiation in our galaxy.
If dark matter is confirmed as the origin of the gamma-ray excess, it would be a groundbreaking discovery, providing critical insights into one of the universe’s most enigmatic components. If pulsars or another astrophysical source are responsible, the findings will still illuminate the extreme environments at the heart of the Milky Way.
Regardless of the outcome, studies like Muru’s underscore the importance of combining advanced simulations with observational data to unravel cosmic mysteries. By reconsidering the Milky Way’s structure and the behavior of dark matter, scientists are moving closer to answering fundamental questions about the universe.
“The galaxy itself may hold vital clues,” Muru said. “Understanding its structure can help us understand the invisible matter that dominates the universe.”
The flattened dark matter model does not close the case, but it adds a crucial piece to the puzzle. For now, the Milky Way’s high-energy glow remains a fascinating mystery — a cosmic beacon pointing scientists toward deeper truths about dark matter, gamma-ray sources, and the complex dynamics of our galaxy’s core.