At the center of almost every large galaxy we observe today lies a black hole of millions to billions of solar masses. The Milky Way's Sagittarius A* is, by these standards, a middling example. The puzzle is this: the earliest known quasars — observed within the first billion years — already contain a billion solar masses. For such structures to exist so soon, black hole seeds had to grow from an extraordinary starting point.

Two main hypotheses compete. The first is that stellar-mass seeds left behind by Pop III supernovae grew step by step through accretion and mergers. This is the slow road, and it struggles to match the quasar timing. The second is the direct-collapse scenario: in certain gas clouds of the early universe, conditions suppressed normal star formation, allowing the gas to collapse straight into a massive black hole. The seed is born already at 10,000 to 100,000 solar masses.

Which path dominated remains contested. JWST has found supermassive black holes existing less than 500 million years after the Big Bang, lending growing support to direct collapse. In either case, something settled into the cores of early galaxies during the universe's first half-billion years — and it has been growing there ever since.