A black hole, 12 billion times as massive as the Sun that grew to gargantuan size in the universe's first billion years, is by far the largest yet spotted from such an early date, researchers said.
The object, discovered by astronomers in 2013, is six times greater than its largest-known contemporaries.
Its existence poses a challenge for theories of the evolution of black holes, stars and galaxies, the scientific journal Nature reported.
Light from the black hole took 12.9 billion years to reach the Earth, so astronomers see the object as it was 900 million years after the Big Bang.
"That is actually a very short time for a black hole to have grown so large," said astronomer Xue-Bing Wu of Peking University in Beijing who led an international collaboration that discovered it.
For its age, this black hole "is really much more massive than anything else we have seen so far", added Christian Veillet, director of the Large Binocular Telescope Observatory in Tucson, Arizona.
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Wu and his colleagues spotted the black hole using the Lijiang Telescope in Yunnan, China.
The object appeared as a bright, red, point-like source.
The brightness and spectrum of its light revealed it to be an ancient quasar: a large black hole that occupies the centre of a galaxy and causes interstellar gas to overheat and shine brighter than any star as it spirals into the hole's gravitational sink.
Supermassive black holes are thought to lurk at the centre of most galaxies, and some have been seen that are as many as 40 billion times the mass of the Sun.
But reaching such sizes involves swallowing interstellar matter and merging with other large black holes, so it takes time.
Until now, the most massive known black hole less than one billion year-old was around two billion solar masses.
Wu's team now plans to make follow-up observations, including some using the Hubble Space Telescope, the report said.