Astronomers said they have found evidence of something they never expected to see – two black holes lurking inside a 12-billion-year-old cluster of tightly-packed stars in our Milky Way galaxy.

The international team was taken by surprise when noting what appeared to be two black holes, each about 10 to 20 times more massive than our sun, near the core of a star cluster named Messier 22 (M22), they wrote in Nature.

The finding challenges much of the accepted science on globular star clusters, which are about 10 billion years old on average and contain around a million stars.

Theorists had believed that no more than one matter-sucking black hole could exist in a globular cluster, of which there are dozens in the Milky Way.

“We were searching for one large black hole in the middle of the cluster, but instead found two smaller black holes a little way out from the centre,” study co-author James Miller-Jones from the International Centre for Radio Astronomy Research said of the “very surprising” find.

This meant that “all the theory and simulations need refinement,” he added.

Simulations of cluster evolution had shown that many black holes are created early in a cluster’s history. Scientists had believed the black holes sink towards the middle of the cluster where they start a gravitational “dance” – most of them being flung out of the cluster with only one remaining.

But the team said their evaluation of X-ray, radio, optical and infrared images of M22 led them to conclude that the objects they spotted were indeed black holes.

Black holes are very dense regions in spacetime with a gravitational force so strong that even light cannot escape. Scientists who study them hope to learn more about the evolution of galaxies.

A black hole is what is left when a massive star dies and collapses in on itself.