Dark Matter May Replace Black Hole at the Milky Way's Core, Astronomers Suggest
New Astronomical Research Challenges Long-Held Assumptions about Sagittarius A*
Astronomers suggest that the heart of the Milky Way may not host a supermassive black hole, but instead a vast concentration of enigmatic dark matter producing an equivalent gravitational pull. They argue that this unseen material—believed to account for most of the universe's mass—can explain both the intense motions of stars just light-hours from the galactic center and the smoother, large-scale rotation of matter across the Milky Way's outer regions.
The findings were published yesterday in Monthly Notices of the Royal Astronomical Society.
Rethinking the Milky Way's Dark Heart
The findings challenge the prevailing view that Sagittarius A* (Sgr A*), the proposed black hole at the center of our galaxy, governs the motion of the so-called S-stars—stellar objects that race around the core at staggering speeds of up to several thousand kilometers per second.
Instead, the international research team proposes an alternative explanation: a particular form of dark matter composed of fermions, or lightweight subatomic particles, capable of forming a distinctive cosmic structure consistent with current observations of the Milky Way's core.
In theory, such a model would generate an ultra-dense, compact core encircled by an extensive, diffuse halo, together behaving as a single, unified structure.
The inner core would be so massive and tightly packed that it could replicate the gravitational influence of a black hole, accounting for the observed orbits of the S-stars as well as those of nearby dust-enshrouded objects known as G-sources.
Gaia's Detailed Map of the Outer Halo
Central to the new research is fresh data from the European Space Agency's Gaia DR3 mission, which has precisely charted the rotation of the Milky Way's outer halo, revealing how stars and gas move far beyond the galactic center.
The observations show a slowdown in the galaxy's rotation—known as a Keplerian decline—which the researchers say can be explained by the outer halo of their dark matter model when combined with the conventional mass of the galactic disc and bulge.
They argue this finding bolsters the fermionic dark matter model by exposing a crucial structural distinction:
- Standard cold dark matter halos extend outward with a long power-law tail
- The fermionic model predicts a more compact halo with tighter outer edges
A Global Collaboration Across Astrophysics
The study is the result of a wide-ranging international collaboration involving:
- Institute of Astrophysics La Plata, Argentina
- International Center for Relativistic Astrophysics Network, Italy
- National Institute for Astrophysics, Italy
- Relativity and Gravitation Research Group, Colombia
- Institute of Physics, University of Cologne, Germany
Connecting the Galaxy's Smallest and Largest Scales
"This is the first time a dark matter model has successfully connected such vastly different scales and orbital behaviours, from modern galactic rotation curves to the motions of stars near the center," said co-author Dr Carlos Argü elles of the Institute of Astrophysics La Plata.
"We are not simply swapping a black hole for another dark object," he added. "We propose that the central supermassive structure and the galaxy's dark matter halo are two expressions of the same continuous substance."
Mimicking a Black Hole's Shadow
Crucially, the fermionic dark matter model had already cleared an important hurdle.
Earlier work by Pelle and Colleagues, also published in Monthly Notices of the Royal Astronomical Society, showed that when an accretion disc illuminates these dense dark matter cores, they produce a shadow-like feature closely resembling the image captured by the Event Horizon Telescope (EHT) for Sagittarius A*.
"This is a pivotal moment," said lead author Valentina Crespi of the Institute of Astrophysics La Plata.
"Our model does more than explain stellar orbits and the Milky Way's rotation," she added. "It is also consistent with the iconic 'black hole shadow' image, as the dense dark matter core bends light so strongly that it creates a central dark region encircled by a bright ring."
Testing the New Dark Matter Scenario
The researchers carried out a statistical comparison between their fermionic dark matter model and the conventional black hole explanation.
The found that while existing observations of stars near the galactic center are not yet precise enough to clearly favour one scenario over the other, the dark matter model offers a single, coherent framework that accounts for:
- Stellar motions near the galactic core
- The observed shadow-like feature
- The Milky Way's large-scale rotation
The study also sets the stage for future tests. More detailed measurements from instruments such as the GRAVITY interferometer on Chile's Very Large Telescope, along with searches for distinctive photon rings—features expected around black holes but absent in the dark matter scenario—will be vital in evaluating the model's predictions.
If confirmed, the findings could fundamentally alter our understanding of the massive object lurking at the heart of the Milky Way.
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