The Galactic Center Excess (GCE) is a broad glow of gamma rays that extends thousands of light years around the Milky Way's core. Researchers have proposed several possible explanations for its origin.

One possibility is self-annihilating dark matter, a hypothetical form of matter that makes up much of the universe's mass but has never been directly detected. Another leading explanation involves a large population of millisecond pulsars, rapidly spinning neutron stars that emit high-energy radiation.

Despite years of study, scientists have not reached a consensus on what is causing the signal.

"Interpreting the signal is particularly difficult because the Galactic Center is an exceptionally bright and crowded region of the gamma-ray sky," explains Florian List, study author and researcher at the University of Vienna.

Machine Learning Adds a Missing Piece

Previous statistical analyses generally favored the pulsar explanation. However, those studies overlooked an important source of information: the energy of each detected gamma-ray photon.

To address this limitation, the research team developed a machine-learning system trained on more than one million simulated gamma-ray observations. For the first time, the method evaluated both the spatial distribution of the signal and the energies of individual photons at the same time.

According to the researchers, incorporating photon energy data significantly changes the results.

Earlier studies suggested the signal came from relatively bright unresolved light sources, often called point sources. The new analysis indicates that if such point sources are responsible, they would need to be extremely faint.

"Our new analysis shows that the sources would have to be so faint that they would be almost indistinguishable from the emission expected from annihilating dark matter," says Nick Rodd, study author and scientist at the Lawrence Berkeley National Laboratory.

The findings also create a challenge for the pulsar explanation. If millisecond pulsars are producing the glow, the study suggests there would need to be at least 35,000 of them packed into the center of the Milky Way. That is far more than the few hundred to few thousand sources assumed in some earlier studies.

Dark Matter Remains in the Running

"The origin of the Galactic Center Excess is one of the longest-running debates in astrophysics," says Florian List. "Our work does not show that dark matter is responsible for the signal. However, it suggests that it is still too early to rule out this possibility."

The new results weaken one of the strongest arguments that had been used against the dark matter explanation. While the research does not provide direct evidence that dark matter is generating the gamma-ray glow, it suggests that dark matter remains a credible possibility.

For now, the source of the Galactic Center Excess remains unknown, ensuring that one of astronomy's most enduring mysteries continues.