Astronomers on Wednesday said they had identified an intermediate class of black hole that could explain how supermassive, light-sucking monsters develop in the heart of galaxies.

Their find -- a black hole more than 500 times the mass of the Sun, on the fringe of galaxy ESO 243-249 -- is reported in the latest issue of Nature, the British-based science journal.

In terms of size, it lies between supermassive black holes, which can be billions of times the mass of the Sun, and relative tiddlers, which are between three and 20 solar masses.

Black holes are among the most powerful forces in the Universe. They are concentrated fields of gravity which are so powerful that nothing, not even light, can escape them.

Stellar-mass black holes are believed to have been created from the death throes of massive stars.

But there is less consensus over how supermassive black holes, which lurk in the centre of galaxies, including the Milky Way, are formed.

"One theory is supermassive black holes may be formed by the merger of a number of intermediate-mass black holes," said lead author Sean Farrell, an astrophysicist at Britain's University of Leicester.

"To ratify such a theory, however, you must first prove the existence of intermediate black holes."

As black holes do not emit light, the existence and size of the intermediate black hole was detected thanks to its "ultra-luminous" X-ray emissions, spotted by Europe's XMM-Newton orbital telescope.