Researchers have modeled a concentration of fermionic dark matter that could explain the Milky Way’s central gravity without a black hole.
At the center of our galaxy, something incredibly heavy is pulling the strings. Stars there swing around it at breakneck speeds, gas heats up and glows, and even light itself seems to bend. For years, astronomers have treated this object as a settled case: a supermassive black hole called Sagittarius A*.
However, a new study suggests we may have been misidentifying the culprit. Instead of a bottomless gravitational pit, the Milky Way’s core could be filled with an exotic, ultra-dense form of dark matter—so compact that it behaves like a black hole without actually being one. If this idea is right, it could change how scientists think about black holes and the role dark matter plays in structuring entire galaxies.A big cosmic disconnectThe long-standing problem in galactic physics is scale. Close to the Milky Way’s center, astronomers track the motion of so-called S-stars, which loop around the unseen central object in extremely tight orbits at speeds of thousands of kilometers per second. These motions demand something both massive and compact. At greater distances from the galactic center, tens of thousands of light-years away, the galaxy behaves very differently. For instance, stars orbit more slowly, and recent measurements from the European Space Agency’s Gaia DR3 mission show that this rotation decreases with distance in a way that resembles classical, Kepler-like motion.Traditionally, these two regions are explained using different ingredients: a supermassive black hole for the center and an extended cloud of cold dark matter for the outskirts. The problem is that these models are not naturally connected. “Such horizon-less configurations can reproduce the relativistic effects measured for S2 orbit, while being part of a single continuous configuration whose extended halo reproduces the latest GAIA-DR3 rotation curve,” the study authors note.Standard cold dark matter halos tend to spread out smoothly, following long tails that do not easily reproduce the detailed shape of the Milky Way’s observed rotation curve, especially the Keplerian decline seen by Gaia. This disconnect is what motivated the researchers to explore a different possibility. A single dark solutionThe research team took a different approach by focusing on fermionic dark matter—particles that obey quantum rules preventing them from being squeezed indefinitely into the same space. In their model, the dark matter particles are assumed to have a specific mass range that allows quantum effects to become important at galactic scales. When enough of this matter accumulates, it forms a stable, extremely dense central core surrounded by a more diffuse halo. This single structure does two jobs at once. The compact core can reproduce the observed orbits of S-stars and nearby dust-enshrouded objects known as G-sources, exerting nearly the same gravitational pull expected from a supermassive black hole. At the same time, the surrounding halo naturally explains the slowing of the Milky Way’s rotation at large distances when combined with the mass of ordinary matter in the galaxy’s disk and bulge. Unlike standard dark matter models, the fermionic halo becomes more compact at large radii, which helps explain why Gaia sees a clear decline in orbital speeds. The team also addressed a crucial observational test, though not within this study alone. In earlier, related work, researchers showed that when hot gas falls toward such a dense dark matter core, the bending of light can produce a dark central region surrounded by a bright ring. This shadow-like feature closely resembles the image of Sagittarius A* captured by the Event Horizon Telescope, even though the dark matter core has no event horizon.The future of the Milky Way’s centerIf the Milky Way’s center is indeed dominated by fermionic dark matter, it would mean that the galaxy’s central object and its dark matter halo are not separate components. “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,” Carlos Argüelles, one of the study authors and a scientist at the Institute of Astrophysics La Plata, said.This would offer one of the first unified explanations linking the extreme gravity near the galactic center with the large-scale structure of the galaxy. It could also influence how astronomers interpret black hole candidates in other galaxies and guide future efforts to identify the true particle nature of dark matter.However, the idea is not yet settled. With current data, the motions of stars near the galactic center can still be explained equally well by a black hole or by a dense dark matter core. The two scenarios begin to differ only in subtle ways. One key distinction is the presence of photon rings—specific light patterns expected around true black holes but absent in dark matter core models.Future observations with instruments such as the GRAVITY interferometer on the Very Large Telescope and improved Event Horizon Telescope will be crucial. “More accurate data, particularly from stars closer to Sgr A, is necessary to statistically distinguish between the models considered,” the study authors said.For now, the study has already shifted the debate—showing that even the best-known structure in our cosmic neighborhood may still be open to reinterpretation. The study is published in the journal Monthly Notice of the Royal Astronomical Society.
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