The black hole idea is actually weirdly solid, its a case of the maths says we definetaly should be but observation and just intuition says its crazy. If you consider the event horizon to be the surface of a volume, black holes get less dense as their radius increases, you can have a black hole with the same density as rock, water, air, even the miniscule density of the gas in a vacuum, so long as teh black hole is large enough. The average density of the observable universe is higher than the density of a black hole the size of the observable universe so technically we should be in one.

Technically this doesn't have to affect anything, larger black holes can have gentler gravity gradients and nothing in physics actually demands all the mass inside be concentrated at a miniscule central point, it just works out that way for black holes of the size we've seen so far. So the entire universe could be a black hole (assuming its finite) with the event horizon just being functionally inacessable and the black hole so large that internal conditions aren't really influenced in any way.