The biggest black holes known are mind-bendingly enormous. The Milky Way’s central black hole, Sagittarius A*, weighs about 4 million suns - already a serious heavyweight. But other galaxies have black holes much, much bigger. The famous M87 black hole, the first one ever photographed, contains the mass of about 6.5 billion suns. Some recently discovered supermassive black holes weigh in at over 20 billion solar masses.
A big mystery is how they got so big. We know stellar black holes form from dead massive stars, but those start at only a few solar masses. Supermassive black holes appear to have existed when the universe was only a few hundred million years old - too soon for them to have grown to billions of suns just by eating things. Most current theories suggest they formed from large “seeds” - possibly clouds of gas that collapsed directly into black holes early in cosmic history.
Once formed, supermassive black holes seem to grow alongside the galaxies they’re in. The size of a galaxy’s black hole and the size of the galaxy’s central bulge are connected in ways scientists are still working out - implying that galaxies and their black holes evolve together over billions of years.
