Here’s two things that we know to be true about black holes.
1) when a massive star dies and goes supernova, the core of the star collapses under gravity to form a black hole that’s heavier than 3 times heavier than the Sun.
2) There’s a supermassive black hole at the centre of every galaxy, every island of stars in our Universe. And they’re anywhere from a million to 10s of billion times heavier than the Sun. But those two facts, that we have lots of evidence to support, means that we have a gap, both a knowledge gap and a mass gap, where we don’t find any black holes between 100 to 100,000 times heavier than the Sun.
The only way we know to make a black hole is through a supernova, making the smaller star mass black holes, so the supermassive ones if they start out that small need to grow slowly through that mass gap until they reach supermassive status.
So why don’t we find any of these intermediate mass black holes...?