system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: why does only empty space expand during cosmic inflationary expansion? Why are occupying objects unaffected from expansion ?

I remember reading a comment on this Subreddit where someone wrote that expansion should not be visualized as the boundary of the universe expanding but as the fundamental physical spacetime fabric itself expanding. When I rescale a picture or blow up a balloon, everything on the surface gets larger. So why is only empty space and distances being enlarged, why not physical objects too?
gpt
Because there are forces that pull in the opposite direction such as gravity, Nuclear Strongforce and electromagnetism. You actually need to get pretty far away before the expansion of the universe is significant enough to not be counter acted by some force. Even Galaxies that are close together like the Milky Way and other galaxies in the Virgo Super cluster are close enough to resist the expansion of the universe.