At the center of nearly every large galaxy sits a supermassive black hole with millions to billions of solar masses. The Milky Way's Sagittarius A* weighs 4 million solar masses. The black hole in M87 weighs 6.5 billion solar masses. TON 618 is one of the most massive black holes ever estimated, with published estimates often quoted in the tens of billions of solar masses.

A Formation Puzzle

How supermassive black holes formed is still debated. They already existed when the universe was less than a billion years old — too soon, it seems, for them to have grown from stellar-mass seeds by simply eating gas. Some may have started as massive seeds that collapsed directly from clouds of primordial gas, skipping the star stage entirely.