> "Couldn't there exist different laws of gravity for big things like planets and small things like rocks, which explains why I've never seen rocks attracted to each other?"

Something very related is a topic that is seriously discussed among physicists:

> https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics

The problem with the existing theories of gravitation which seem to work quite well on the scale of solar systems is that there does seem to exist evidence that some observed data on the scale of galaxies might not be well-explainable by these theories. See

> https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics#Ob...

Very recently new evidence for MOND came up:

> https://phys.org/news/2023-08-smoking-gun-evidence-gravity-g...

> https://news.ycombinator.com/item?id=37051241

See also:

> https://tritonstation.com/2023/08/02/is-ngc-1277-a-problem-f...

> https://news.ycombinator.com/item?id=37009037