That is a very good question which I've been thinking about for years: How is it that at cosmological scales it is reasonable (apparently) to assume that the ma

That is a very good question which I've been thinking about for years: How is it that at cosmological scales it is reasonable (apparently) to assume that the matter density of the universe is homogenous (i.e. the same everywhere) in space, yet at a local level, we have Schwarzschild/Kerr spacetimes around every spherically symmetric body (whether black hole, star, planet or atom)? How does a homogeneous universe emerge from this "bubbly" spacetime, as you call it, at larger scales? Unfortunately, Einstein's field equations are not linear, so in contrast to other (linear) field theories, this case is not as simple as superposing several black hole solutions to a global solution and then averaging or zooming out in an appropriate way, since the sum of two solutions won't give another solution. I'm wondering whether anyone has ever looked into the scaling behavior of the Einstein field equations but the answer from most people in the community that I've talked to has been no.