Dark matter could be masquerading as a black hole at the Milky Way’s core

This unseen material, which makes up most of the universe’s total mass, may be able to explain two very different observations at once. Near the galaxy’s center, stars move in fast, chaotic paths just light hours (often used to measure distances within our own solar system) from the core. Farther out, stars and gas rotate more smoothly across the vast outer regions of the Milky Way.

The findings were published in Monthly Notices of the Royal Astronomical Society (MNRAS).

Challenging the Black Hole Explanation

For decades, scientists have believed that Sagittarius A* (Sgr A*) is a supermassive black hole responsible for the extreme orbits of a group of stars known as the S stars. These stars race around the galactic center at speeds reaching several thousand kilometres per second.

The new study questions that interpretation. The research team proposes that a specific form of dark matter made of fermions, which are lightweight subatomic particles, could instead form an unusual cosmic structure that fits what astronomers observe at the Milky Way’s core.

A Dark Matter Core and Halo

According to the model, this fermionic dark matter would naturally form a very dense and compact central core, surrounded by a much larger and more diffuse halo. Together, the core and halo would behave as a single, continuous system.

The inner core would be massive and concentrated enough to closely imitate the gravity of a black hole. This could explain not only the paths of the S stars, but also the motion of nearby dust covered objects called G sources that orbit close to the galactic center.

Evidence From the Galaxy’s Outer Regions

A key piece of evidence comes from new observations by the European Space Agency’s GAIA DR3 mission. This survey precisely mapped how stars and gas move in the outer halo of the Milky Way, revealing the galaxy’s rotation curve in unprecedented detail.

The data show a slowing in orbital speeds at great distances from the center, a pattern known as the Keplerian decline. Researchers say this behavior matches predictions from the dark matter halo in their model when combined with the known mass of the Milky Way’s disk and central bulge.

They argue this strengthens the fermionic dark matter explanation. Standard Cold Dark Matter models predict halos that extend outward with a long power law tail. In contrast, the fermionic model produces a more compact halo with tighter outer edges.

An International Collaboration

The research was carried out by scientists from institutions in several countries, including the Institute of Astrophysics La Plata in Argentina, the International Centre for Relativistic Astrophysics Network and the National Institute for Astrophysics in Italy, the Relativity and Gravitation Research Group in Colombia, and the Institute of Physics University of Cologne in Germany.

“This is the first time a dark matter model has successfully bridged these vastly different scales and various object orbits, including modern rotation curve and central stars data,” said study co author Dr. Carlos Argüelles of the Institute of Astrophysics La Plata.

“We are not just replacing the black hole with a dark object; we are proposing that the supermassive central object and the galaxy’s dark matter halo are two manifestations of the same, continuous substance.”

Matching the Black Hole Shadow

The model had already cleared an important hurdle. In an earlier study by Pelle et al. (2024), also published in MNRAS, researchers showed that when an accretion disk shines light onto these dense dark matter cores, the result is a shadow like feature. Remarkably, this shadow closely resembles the image captured by the Event Horizon Telescope (EHT) for Sgr A*.

“This is a pivotal point,” said lead author Valentina Crespi of the Institute of Astrophysics La Plata.

“Our model not only explains the orbits of stars and the galaxy’s rotation but is also consistent with the famous ‘black hole shadow’ image. The dense dark matter core can mimic the shadow because it bends light so strongly, creating a central darkness surrounded by a bright ring.”

What Future Observations May Reveal

The team compared their fermionic dark matter model directly with the traditional black hole explanation using statistical methods. While existing data on stars near the center cannot yet clearly favor one scenario over the other, the dark matter model offers a single framework that explains both the galactic center (central stars and shadow) and the broader structure of the galaxy.

Future observations could help settle the debate. More precise measurements from tools like the GRAVITY interferometer on the Very Large Telescope in Chile, along with searches for photon rings, could provide decisive evidence. Photon rings are expected around true black holes but would not appear in the dark matter core model.

If confirmed, these results could significantly change how scientists understand the massive object shaping the heart of the Milky Way.