Is our galaxy’s black hole really made of dark matter?

At the center of our galaxy lies a supermassive black hole called Sagittarius A* — but one group of researchers suggests it may not be a black hole at all. The team says that it and other black holes around its size may actually be clumps of dark matter.

Dark matter, so named because it does not appear to interact with light or ordinary matter in any way other than gravitationally, makes up about 85 percent of the total matter in the universe, but we know very little about it. What we do know, due to the way galaxies rotate, is that most galaxies are embedded in halo matter. “We know it must be at the edges of galaxies, but we don’t know what’s going on in the very center,” he says Valentina Crespiová at the National University of La Plata (UNLP) in Argentina.

Crespi and his colleagues built a model of the galactic core made of dark matter in the form of extremely light particles called fermions. They found that fermionic dark matter can form a clump so massive and dense that it can look almost exactly like a supermassive black hole from a distance.

“From Earth, you would see something very similar to what you would see in a black hole scenario – but if we were to drive a ship towards the center, we would be able to pass through without a problem,” he says. Carlos Arguelles at UNLP, who was part of the research group. “You won’t die by being eaten by a black hole, you’ll make it through peacefully.”

Of course, we don’t have the ability to actually send a ship through the center of the galaxy, so the team’s model is based mostly on the orbits of stars and small clouds of gas near Sagittarius A*. It also matches measurements of the rotation of the entire galaxy, as well as an image of Sagittarius A* released by the Event Horizon Telescope (EHT) in 2022. The image shows a glowing ring of superheated matter around the black hole, which could also be caused by the gravitational pull of the dark matter core.

But just because the idea that Sagittarius A* is made of dark matter fits the observations doesn’t mean it’s true. “Based on the fact that it’s a simpler answer that fits the evidence, I personally believe that the celestial body at the center of our galaxy is very likely a black hole,” he says. Gaston Giribet at New York University. “However… all options must be analyzed and this one is certainly an interesting one.

One potential problem is that while the dark matter core matches the orbits of objects a few light-hours from the edge of the black hole, known as the event horizon, it’s unclear whether the model works for observations “at the very threshold of the event horizon,” he says. Shep Doeleman at Harvard University, who is the founding director of the EHT project. In particular, the spiral pattern of the magnetic fields in the region seems consistent with a black hole, he says.

Another problem is that fermionic dark matter could not form a cluster larger than about 10 million times the mass of the Sun. In the abstract, this might seem like a positive: fermionic clumps of dark matter could get around this matter and then collapse into black holes, which could explain the ongoing mystery of how supermassive black holes grew so large. But an EHT image of a much larger supermassive black hole called M87* looks almost identical to Sagittarius A*, even though M87* has about 6.5 billion solar masses, which might make the idea harder to accept.

Researchers admit that a dark matter core is no more likely than a black hole, and in fact may be less likely. “Today, with the instruments available, it is not yet possible to distinguish 100 percent whether it is really dark matter or not,” says Crespi. To do that, we’d need images of such high resolution that even the next generation of EHTs almost certainly won’t come close, Argüelles says—it’ll be decades before we can say for sure, if not longer.

If Sagittarius A* is dark matter, this will be extremely important. Fermionic dark matter is not predicted by the current standard model of cosmology, which favors heavier, slower-moving particles as dark matter candidates, so its relatively nearby core would shake up our understanding not just of black holes, but of the universe as a whole.

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