For several decades, the orbital mechanics of celestial bodies in proximity to our Milky Way Galaxy’s core have been widely considered definitive evidence for a supermassive black hole. However, Dr. Valentina Crespi, affiliated with the Institute of Astrophysics La Plata, and her collaborators propose an alternative hypothesis: a distinct type of compact entity, composed of self-gravitating fermionic dark matter, could account for these observed stellar trajectories with equal plausibility.
A compact object constructed from self-gravitating fermionic dark matter. Image credit: Gemini AI.
The prevailing cosmological paradigm posits that Sagittarius A* — a theoretical supermassive black hole situated at the Galaxy’s center — is the gravitational influence dictating the orbits of a specific stellar population, designated as S-stars. These stars traverse their paths at extraordinary velocities, reaching speeds of up to several thousand kilometers per second.
Dr. Crespi and her research associates have advanced a novel proposition, suggesting that a particular form of dark matter, comprised of fermions or exceptionally lightweight subatomic particles, could coalesce into a singular cosmic formation that aligns with our current understanding of the Milky Way’s central region.
In theory, this formation would manifest as an exceedingly dense, compact nucleus enveloped by an expansive, diffuse halo, functioning collectively as a unified structure.
This proposed ultra-compact and massive object at the Milky Way’s nucleus could exert a gravitational pull comparable to that of a black hole, thereby elucidating the orbital patterns of the S-stars documented in prior investigations, as well as those of the dust-obscured entities referred to as G-sources, which are also found in the vicinity.
Crucially, the recent data acquired from ESA’s Gaia DR3 mission holds significant relevance for this new research. This mission has meticulously charted the rotational velocity profile of the Milky Way’s outer halo, detailing the orbital behavior of stars and gas far from the galactic center.
The observed deceleration in our Galaxy’s rotational curve, known as the Keplerian decline, is adequately explained by this proposed dark matter model’s outer halo, when integrated with the conventional disk and bulge mass components formed from ordinary matter.
This finding serves to bolster the credibility of the fermionic model by drawing attention to a key distinction in its structure.
Whereas conventional Cold Dark Matter halos tend to disperse with an extended ‘power law’ tail, the fermionic model predicts a more tightly bound configuration, resulting in more condensed halo extremities.
“This represents the inaugural instance where a dark matter model has successfully harmonized observations across such disparate scales and diverse celestial object orbits, encompassing contemporary rotational curves and data pertaining to central stars,” remarked Dr. Carlos Argüelles, also a member of the Institute of Astrophysics La Plata.
“Our proposition extends beyond merely substituting the black hole with a dark object; we are advancing the notion that the supermassive central entity and the Galaxy’s dark matter halo are, in fact, two observable facets of the same continuous substance.”
Significantly, the team’s proposed dark matter model based on fermions had previously undergone a critical validation.
A study published in 2024 demonstrated that when an accretion disk illuminates these dense dark matter cores, they generate a shadow-like feature strikingly reminiscent of the one captured by the Event Horizon Telescope (EHT) collaboration for Sagittarius A*.
“This is a pivotal moment. Our model not only accounts for stellar orbits and galactic rotation but also aligns with the renowned ‘black hole shadow’ imagery,” stated Dr. Crespi.
“The dense dark matter core can effectively mimic such a shadow due to its profound light-bending capabilities, thereby creating a central dark region encircled by a luminous ring.”
The astronomers conducted a statistical comparison between their fermionic dark matter model and the conventional black hole paradigm.
Their findings indicate that while current data concerning stars in the inner galactic region cannot definitively differentiate between the two hypotheses, the dark matter model offers a cohesive theoretical framework that accounts for both the galactic center (including central stars and the shadow phenomenon) and the galaxy as a whole.
“Acquiring more refined observational data from sophisticated instruments, such as the GRAVITY interferometer situated on ESO’s Very Large Telescope in Chile, and the pursuit of the distinctive signature of photon rings — a key characteristic of black holes and absent in the dark matter core scenario — will prove essential in verifying the predictions of this novel model,” the researchers stated.
“The ultimate resolution of these investigations holds the potential to fundamentally alter our conceptualization of the nature of the colossal cosmic entity residing at the heart of the Milky Way.”
The research conducted by the team was officially published today in the esteemed journal, Monthly Notices of the Royal Astronomical Society.
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V. Crespi et al. 2026. The dynamics of S-stars and G-sources orbiting a supermassive compact object made of fermionic dark matter. MNRAS 546 (1): staf1854; doi: 10.1093/mnras/staf1854
