What do quarks break down to? Nothing, so far as we know, they are fundamental particles, not made of anything else. What determines their density, what's 'holding them up' and preventing them from shrinking further? We don't know. So this is where our knowledge of the quantum world stops. We don't know what's inside a black hole. It's not atomic scale, it's not even subatomic scale, it's something past that. Our theories offer no clue of what that something looks like, but we know it exists. It curves space-time, it produces gravity.