system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: The accelerating expansion of the universe means other galaxies are moving away from ours quickly, but why is nothing within our own galaxy expanding, like the distance between the earth and the sun?

Or why is that that the expansion is between galaxies and not between every single point in the universe? What's special about galaxies?
gpt
That's actually really simple:

Gravity is stronger than the expansion of the universe at relatively small distances. 

That is, gravity is sufficient to keep our planet, solar system, and galaxy from responding to the expansion of the universe. It's like a leash that keeps everything tied together.

However, at larger distances, gravity is *not* strong enough; it's a field force that obeys the inverse-square law, which means that it gets much weaker as the distance between two objects increases. 

So, the force between galaxies is not enough to hold those galaxies together--but the force within a galaxy *is*.