Yeah I figured it maybe also had something to do with the distribution of matter throughout the universe. We assumed when we made predictions of the distant past that the supermassive black holes at the center of galaxies were the same mass as they are today, but if they were less massive then it might help explain why black holes didn't gather as much material into it's orbit as we would have thought.

I think you're right though that it has more to do with the negative pressure that space and the black holes seem to exert although I must admit I don't really understand what that means or how you would get a negative pressure from a black hole or from space.