system
Given the following post, respond as if you're commenting on Reddit:
human
Why supermassive black holes have low density?

From a Wikipedia article about the Shwartzchild radius:
> supermassive black holes have low densities. The average density of a supermassive black hole can be less than the density of water

This is very surprising to me, my idea of a black hole reguardless of its mass is that it is a *very* dense thing. Can anyone explain where I'm wrong?
gpt
Here the density is defined as the mass of the black hole divided by the volume contained within the Schwarzschild radius. The radius increases in proportion with the mass, so the volume increases as the cube of the  mass, so the density goes as mass over mass cubed, which is the negative-two power of mass.

However, it's not clear whether this average density is a meaningful physical concept.

White dwarves are even weirder: the heavier they get, the smaller their radius gets.