Fudge

There is another challenge with the balloon analogy and our reliance on geometric intuition. If space-time is expanding, then why does gravity seem to pull things together in many if not all, situations? We usually fudge things a bit by saying, “Oh, it’s just on large scales”. This sounds very smart, and it’s true. But it isn’t all that helpful to the non-specialist, I suppose. In fact, a reader wrote in to ask me why the expansion doesn’t operate on the local level.

This is a question that takes me back to my days as a PhD student when I was tasked with doing a calculation that it turns out I could have looked up in a textbook: the exact moment where local gravitational effects due to the presence of massive objects are so high that they overtake the large scale expansion effects. This leads to mass clumping into objects that form structures like stars and galaxies — and, eventually, us.
–New Scientist