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

A team led by astronomers at the University of California, Berkeley, discovered the two gigantic black holes in clusters of elliptical galaxies more than 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.

In research released on Monday by the journal Nature, the scientists suggest these black holes may be the leftovers of quasars that crammed the early universe. They are similar in mass to young quasars, they said, and have been well hidden until now. They used ground-based telescopes and the Hubble Space Telescope for the job, as well as supercomputers in Texas.

A black hole is formed by the collapse of a super-size star. It's a region where nothing, not even light, can escape. Most if not all galaxies are believed to have black holes at their center; the bigger the galaxy, it seems, the bigger the black hole.

Quasars are some of the most energized and distant of galactic centers.

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

Astrophysicist Chung-Pei Ma, part of the Berkeley team, speculates the black holes remained hidden for so long because they are living in quiet retirement.

"For an astronomer, finding these insatiable black holes is like finally encountering people nine feet (2.75 meters) tall whose great height had only been inferred from fossilized bones. How did they grow so large?" Ma said in a news release. "This rare find will help us understand whether these black holes had very tall parents or ate a lot of spinach."

Oxford University astrophysicist Michele Cappellari, who wrote an accompanying commentary in the journal, agreed that the two newly discovered black holes "probably represent the missing dormant relics of the giant black holes that powered the brightest quasars in the early universe".

One of the newly detected black holes is 9.7 billion times the size of the sun. The second, slightly farther from Earth, is as big or even bigger.