Black hole stars could explain otherwise impossibly large black holes.
Black hole stars are a hypothesis which explain why we see black holes from the first billion years with hundreds of millions of solar masses, which should not be possible. The stars we can see max out around 300 solar masses and can create black holes of dozens of solar masses. They can only grow very slowly because matter is super heated on a very long orbit before reaching the event horizon, which in turn puts pressure on other matter to keep it away. Even with merging black holes, there would not have been enough time to grow this large.
Black hole stars resolve this by being so large—forming in dark matter halos from many millions of solar masses of material which outweighs smaller galaxies—that when the pressure causes the core to collapse into a black hole, the resulting supernova is not powerful enough to blow away the outer mantle. The outer gravity overwhelms the black hole’s outward forces, force feeding it and generating tremendous radiation pressure which keeps the star stable until it runs low on matter, at which point it would explode.rK5