At the heart of the Milky Way, something massive is warping space itself. For decades, the presumed culprit has been Sagittarius A*, a supermassive black hole with four million times the sun’s mass. Stars whip around it at thousands of kilometres per second, their orbits testifying to something extraordinarily dense lurking there. But now astronomers are asking: what if it’s not a black hole at all?
A new study published today in Monthly Notices of the Royal Astronomical Society proposes something rather different: a colossal clump of invisible dark matter producing precisely the same gravitational effects. Not a cosmic imposter, exactly, but an entirely different beast that might reshape our understanding of what anchors galaxies together.
The research comes from an international team led by Valentina Crespi at the Institute of Astrophysics La Plata in Argentina, working with colleagues across Italy, Colombia, and Germany. They’ve built a model using fermions, light subatomic particles that could theoretically form the mysterious dark matter making up roughly 85 per cent of the universe’s mass. In their model, these particles create a super-dense core surrounded by a vast, diffuse halo, the whole thing acting as a unified structure that could explain both the violent stellar ballet near the galactic centre and the gentle rotation of matter hundreds of light-years away.
What makes this work compelling is its scope. “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,” says study co-author Carlos Argüelles, also at La Plata. Previous attempts to replace black holes with alternative theories typically failed on one scale or another; they might explain distant stellar orbits but couldn’t account for the rapid S-stars orbiting within light-hours of the centre, or vice versa.
The fermionic dark matter model manages both. The team used astrometric data from the S-stars (particularly S2, which completes an orbit every sixteen years) along with observations of the G-sources, dust-enshrouded objects in similar orbits. Crucially, they also incorporated data from the European Space Agency’s GAIA DR3 mission, which has meticulously mapped how stars move in the Milky Way’s outer halo. The mission observed a characteristic slowdown in our galaxy’s rotation curve at large distances, a pattern the researchers say fits their dark matter model when combined with ordinary matter in the galaxy’s disc and bulge.
The model predicts something rather interesting about structure. Whilst traditional Cold Dark Matter halos spread out following an extended power law tail, the fermionic model produces tighter, more compact halo tails. It’s a subtle difference, perhaps, but one that could eventually distinguish between theories as observations improve.
There’s another test the model has already passed, though; one that might have seemed insurmountable. In 2022, the Event Horizon Telescope collaboration released an image of Sagittarius A* showing a dark central region surrounded by a bright ring. A previous study by Pelle and colleagues, also published in Monthly Notices, demonstrated that when an accretion disc illuminates a sufficiently dense fermionic dark matter core, it casts a shadow-like feature strikingly similar to what the EHT imaged. “This is a pivotal point,” Crespi says. “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.”
The researchers used Bayesian statistics to compare their fermionic model with the traditional black hole scenario. Current data for the inner stars can’t yet decisively distinguish between the two, they found, but the dark matter model provides what Argüelles describes as a unified framework. “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.”
That’s quite a claim. One object, two scales.
Future observations should help settle matters. The GRAVITY interferometer on the Very Large Telescope in Chile can provide more precise measurements of stellar orbits. And there’s a unique signature to search for: photon rings, a key feature of black holes that should be absent if the galactic centre harbours a dark matter core instead. The distinction might sound subtle, but it represents fundamentally different physics.
Whether or not the Milky Way’s centre turns out to contain dark matter rather than a black hole, the research demonstrates something valuable (that alternatives to black holes needn’t be vague hand-waving but can make testable, quantitative predictions across vastly different scales). Sometimes in science, the most productive questions aren’t about proving something right, but about proving we haven’t yet ruled out the alternatives.
Study link: https://academic.oup.com/mnras/article/546/1/staf1854/8431112
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