Massive black holes could be the source of dark energy and the accelerating expansion of the universe, according to observations of ancient, dormant galaxies with black holes at their centre.

The laws of physics suggest that gravity should cause the universe to contract, but a mysterious force, which physicists call dark energy, seems to be counteracting this and making the universe expand at an accelerating rate.

One possible explanation is that the source of this dark energy is black holes, but there hasn’t been good experimental evidence to conclusively support this idea.

Chris Pearson at the Rutherford Appleton Laboratory in Harwell, UK, and his colleagues compared groups of galaxies with black holes at their centre: a young, distant group and a closer, older group that have stopped growing. They calculated that the black holes grew in mass by seven to 20 times, which can’t be fully explained by the absorption of stellar material or mergers with other galaxies.

Instead, Pearson and his team tried to account for the growth by proposing that it is related to the universe’s accelerating expansion. “When we model that into what we see, we can actually explain the observations,” says Pearson. “We can see that, in addition to these astrophysical processes for black hole growth, we can explain away this discrepancy in the mass growth by adding in the fact that they may contain dark energy and they’re coupled to the expansion of the universe.”

The model they used involved an interpretation of Albert Einstein’s general theory of relativity that says that black holes contain vacuum energy, a kind of energy that exists in space everywhere due to quantum particles popping in and out of existence. “When we did the sums, we found that these black holes might actually be able to explain the entirety of what’s required to balance the universe with this dark energy,” says Pearson.