Astronomers said Wednesday they had detected an "impossibly large" and ancient black hole that challenges theories about how these phenomena grew in the early Universe.

With a mass 12 billion times that of our Sun, the black hole formed about 900 million years after the Big Bang that created the Universe 13.7 billion years ago -- a very short period in galactic history, the surprised team reported in the journal Nature.

"Forming such a large black hole so quickly is hard to interpret with current theories," study co-author Fuyan Bian of the Australian National University said in a statement.

Matter-sucking black holes are extremely dense regions in spacetime with a gravitational force so strong that even light cannot escape.

They grow in size by feasting on stars and other matter around them, releasing energy that can be seen from Earth as the bright objects called quasars.

This particular black hole lay at the heart of an ultra-luminous quasar -- the brightest object yet discovered from that period, the team said.

They had picked the quasar to look at because of its unusual red colour, from a survey of over 500 million objects in the northern skies.