> The universe is inside a black hole.

This is trivially true, btw. The Schwarzschild radius of the observable universe is less than the presumed plausible size of the universe as a whole. Or alternatively, a "black hole" is a region of space that nothing can escape, not even light, and ongoing and accelerating spatial inflation causes that to be true on an even smaller scale -- most of the universe is unreachable by light emitted today.

> Dark matter is matter which is outside the black hole, which explains why it has mass but no other observable properties

Except that it interacts as if it was superimposed over objects in this universe, not from outside of it.