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.
But they could not believe their eyes – and the scientific community spent two decades before they accepted what they were seeing.
The previous black hole record-holder was the size of 6 billion suns.
The Oxford University astrophysicist Michele Cappellari, who wrote an accompanying commentary to the research published in the journal Nature, said the findings were at first unbelievable.
“It took a couple of decades to believe that these black holes weren’t just fantasy but actually reality”, he told Radio Four’s Today programme.
In the research, 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.
The scientists used ground-based telescopes as well as the Hubble Space Telescope and Texas supercomputers, observing stars near the black holes and measuring the stellar velocities to uncover these vast, invisible regions.
Black holes are objects so dense that nothing, not even light, can escape. Some are formed by the collapse of a supersize star.
It’s uncertain how these two newly discovered whoppers originated, said Nicholas McConnell, a Berkeley graduate student who is the study’s lead author.
To be so massive now means they must have grown considerably since their formation, he said.
Most if not all galaxies are believed to have black holes at their centre.
The bigger the galaxy, it seems, the bigger the black hole.
Quasars are some of the most energised and distant of galactic centres.
The researchers said their findings suggest differences in the way black holes grow, depending on the size of the galaxy.
“They are monstrous,” Berkeley astrophysicist Chung-Pei Ma told reporters. “We did not expect to find such massive black holes because they are more massive than indicated by their galaxy properties. They’re kind of extraordinary.”
Ma speculates these two black holes remained hidden for so long because they are living in quiet retirement – much quieter and more boring than their boisterous youth powering quasars billions of years ago.
“For an astronomer, finding these insatiable black holes is like finally encountering people nine feet tall whose great height had only been inferred from fossilised 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.”
One of the newly detected black holes weighs 9.7 billion times the mass of the sun. The second, slightly farther from Earth, is as big or even bigger.
Even larger black holes may be lurking out there. Ma said that’s the million-dollar question: How big can a black hole grow?
The researchers already are peering into the biggest galaxies for answers.
“If there is any bigger black hole,” Ma said, “we should be able to find them in the next year or two. Personally, I think we are probably reaching the high end now. Maybe another factor of two to go at best.”