Scientists have found the biggest black holes known to exist - each one 10 billion times the size of our sun.

A team led by an astronomer at the University of California at Berkeley discovered the two gigantic black holes in clusters of galaxies 300 million light years away. That's relatively close on the galactic scale.

The previous black hole record-holder is as large as 6 billion suns. A black hole is formed by the collapse of a super-size star. It's a region where nothing, not even light, can escape.

The scientists say their findings suggest differences in the way black holes grow, depending on the size of the galaxy.

The research was released Monday by the journal Nature.