My wife and I do things in threes: three taps, three nudges, three blinks, three noises, whatever. It means “I love you”. It’s a nice way to say it when you’re too tired to say it. I think it originated when we’d say it as we were falling asleep.

Yes, gravity is a theory. In scientific terms, theories are proven, repeatable, and accepted. It’s the most robust and strongest form of scientific “fact” we have (since new discoveries can change our understanding, we can’t honestly declare it indisputably factual).

When people say “I have a theory…” they usually mean “I have a hypothesis…” which is some idea or problem statement that is unproven, untested, unverified.

OP was playing off the conflation of those two things.