Astronomers say they have found possibly the biggest black hole ever observed, a leviathan with a mass 17 billion times that of the sun, brooding at the heart of a distant galaxy.

The black hole is as unexpected as it is vast, for it accounts for nearly a seventh of its galaxy's mass, a finding that may rewrite theories of cosmic formation, they said.

Biggest ever? ... an artist's impression of a black hole.

Named NGC 1277, the monster lies 220 million light years away in a small galaxy just a tenth the size of our Milky Way.

The hole's maw is more than 11 times wider than Neptune's orbit around the sun.