I don't see the differences between our understanding of either.

We really do understand the deformation of rubber in terms of local molecular interactions. Not in analogies but in actual detail of what the molecules are doing.

Yes, but what are molecules? Why do they exist and have the strong/weak/electric forces? And why do those forces ...?

No matter how many levels you go down and say, "Ah, X results from the effects of Y", you're still doing the exact same thing you (and PhilGoetz) claim is going on with gravity: you're "passing the buck" to another hypothetical entity.

I don't believe this distinction is useful. Rubber is no more explained when you know it's "really" just molecular forces writ large, than when you merely knew how it works.

The only way to have a terminating procedure to determine when you understand it is when you can predict your observations of it in a model that connects to your model for everything else. Positing the existence of molecules only helps you to the extent that helps generate such a model.

So, I think that both gravity and chemicals are equally well explained: we have a model that works for both.