They are still struggling to explain why but they speculate that the light emanates from gas that is being ripped apart as it disappears into the black hole.

Until now the only black holes showing colour were blue

"We think that the violent swirling motion of the gas around the black hole might be acting as a natural particle accelerator," says Dr Paul Francis, of the Australian National University.

"This generates an intense beam of sub-atomic particles and, somehow, this beam produces the strong pink glow as it blasts out into space," he said.

Black holes are super-dense astronomical objects whose gravitational pull is so intense that even light is dragged in. To have the same density as a black hole, the Earth would have to be compressed to smaller than a pinhead.

So black holes themselves cannot be coloured but the destruction they cause nearby can create light.

"We're pretty certain it isn't the black holes themselves that are pink," said Dr Francis. "We think that these black holes are devouring anything that comes near them. Possibly as the mangled remains of space matter, stars and gas clouds swirl down the throat of the black holes, they emit an intense pink light."

Paul Francis has found about 100 pink black holes

Though the mysterious pink glow was not hard to spot, identifying the objects was more difficult. "These pink things were quite easy to find," said Dr Francis.

"The hard bit was proving that they are black holes. They are more than a billion light-years away, and are more than one hundred thousand times fainter than the human eye can see. It took the combined power of four of Australia's best telescopes to identify what they were," he said.

It is well known that massive black holes devour stars and gas. Black holes like this are called quasars, and were first discovered in the 1960s. But until now only blue quasars had been seen.

An early theory that a shroud of dust made blue quasars look pink has now been discounted. Dust clouds absorb blue light, leaving a yellow-red glow.