Dark matter could provide the brakes needed to bring supermassive black holes together on the long, spiraling journey to their doom. A puzzle known as the last parsec problem could be solved by allowing dark matter particles that remain clustered around black holes and interact with each other to travel the final distance between them, according to new mathematical modeling.

This discovery suggests that the mysterious matter that gives the universe its extra gravity must be able to interact with itself, as the problem cannot be solved by non-interacting dark matter models.

“We show that including the previously overlooked effect of dark matter can help supermassive black holes overcome this final parsec of separation and merger,” says Gonzalo Alonso-Álvarez, a physicist at the University of Toronto and McGill University. “Our calculations explain how this could happen, contrary to what was previously thought.”

Supermassive black holes, which reside at the hearts of galaxies, present a great mystery to astronomers. We know that smaller black holes form from the collapsed cores of massive stars that ran out of fusion fuel and were ejected from the cosmic bucket. These smaller masses can coalesce into larger masses; the largest black hole merger discovered to date produced an object with a mass equivalent to 142 Suns.

Supermassive black holes are millions to billions of times the mass of the Sun. It’s reasonable to assume that they could grow that big by merging with other monster-sized black holes. Throughout the history of the universe, we’ve even seen supermassive black holes orbiting each other, with galaxies likely heading toward an eventual collision after merging.

But it’s not clear how these supermassive black holes collide. Models suggest that as they orbit each other, they transfer their orbital energy to the stars and gas around them, causing their orbits to shrink. As their distance decreases, fewer things are able to steal their momentum.

Once they are about a parsec (about 3.2 light-years) away, their galactic neighborhood can no longer support further orbital decay, so the black hole’s orbit remains stable for a very long time. How long? At least longer than the universe has existed.

One way to determine whether supermassive black holes have actually merged in the past involves gravitational waves—huge ripples in the fabric of space-time caused by large masses as their speeds change. If supermassive black holes collide in the universe, there should be a characteristic background “hum” of very low-frequency gravitational waves constantly rippling through the universe.

We’ve finally discovered the background hum of a gravitational wave, which means we’ve been missing a key part of the supermassive black hole collision story.

This is the last parsec problem.

Dark matter may be what we’re missing. However, according to previous models of supermassive black hole mergers, their gravitational interaction should also expel dark matter particles from the system, or they would absorb the last of their orbital energy.

The problem with dark matter is that we don’t know what it is. It doesn’t interact with the normal matter of the universe except through gravity, which makes it extremely difficult to study. We actually call it dark matter as a placeholder term, and scientists try to understand its properties by studying the behavior of the universe in other ways.

Alonso-Alvarez and his colleagues wondered whether we might be too hasty in dismissing dark matter as a solution, and they developed mathematical models to test it. They found that interacting dark matter can stay close to the merging supermassive black holes, giving them something to transfer their final orbital energy to, so they can eventually embrace each other to form an extremely large supermassive black hole.

Right now, the results are highly theoretical, but they include observable predictions. For example, the findings show a softening of the gravitational wave background noise, hints of which have already been detected. The results could also be used to understand the dark matter halos surrounding galaxies in the universe, since particles need to interact on a galactic scale to solve the last parsec problem.

Ultimately, the researchers say their findings represent a new tool for unraveling the mysteries of dark matter.

“Our work is a new way to help us understand the particle nature of dark matter,” Alonso-Álvarez says. “We found that the evolution of black hole orbits is very sensitive to the microphysics of dark matter, meaning we can use observations of supermassive black hole mergers to better understand these particles.” The study was published in: Physical Review Letters.