Are galactic black holes actually made of dark matter?

At the center of our galaxy lies a supermassive black hole called Sagittarius A* – but a group of scientists suggest 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 except gravitationally, makes up about 85 percent of the total matter in the universe, but we know very little about it. What we do know, because of the way galaxies rotate, is that most galaxies are embedded in a halo of matter. “We know it must be on the outskirts of galaxies, but we don’t know what happens in the center,” says Valentina Crespi of the National University of La Plata (UNLP) in Argentina.

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

“From Earth, you would see something very similar to what you would see in the black hole scenario – but if we went in a ship towards the center, we could go through without a problem,” says Carlos Argüelles of the UNLP, who was part of the research team. “You will not die from being eaten by the black hole; you will pass 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 largely based on the orbits of stars and small gas clouds near Sagittarius A*. It is also consistent with measurements of the rotation of the entire galaxy, as well as the 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 may also be caused by the gravitational pull of a dark matter core.

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

A potential problem is that even if a core of dark matter matches the trajectories of objects several light-hours away from the edge of the black hole, known as the event horizon, it is unclear whether the model works for observations “right on the doorstep of the event horizon,” says Harvard University’s Shep Doeleman, who is a founder of the EHT project. In particular, the spiral pattern of the magnetic fields in that area seems consistent with a black hole, he says.

Another problem is that fermionic dark matter could not form a clump 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 can grow so large and then collapse into black holes, which could explain the enduring mystery of how supermassive black holes grew so large. But the EHT image of a much larger supermassive black hole called M87* looks almost identical to Sagittarius A*, even though M87* is about 6.5 billion solar masses, which may make the idea harder to accept.

The researchers admit that a dark matter core is no more likely than a black hole, and may actually be less likely. “Today, with the available instruments, it is not yet possible to 100 percent discriminate whether it is really dark matter or not,” says Crespi. To do that, we need images with such high resolutions that even the next generation of EHT will almost certainly not come close, says Argüelles – it will be decades before we can say for sure, if not longer.

If Sagittarius A* is dark matter, however, it will be enormously 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 a core of it relatively nearby would shake up our understanding of not just black holes, but the entire universe.

Mysteries of the Universe: Cheshire, England

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