Do you mean hypothesis? A theory is a well-substantiated explanation based on evidence...and has undergone some kind of rigorous testing/verification. A hypothesis, on the other hand, is "just" a proposed explanation/prediction based on little (or no?) evidence...which is then tested through experiments, etc.
A hypothesis is a proposition. A theory is a model. The two are different concepts but neither implies rigor or lack of rigor. It’s harder to build a complete model from nothing but it happens all the time.
Theories about the universe as a whole (or anything outside the solar system at this point) can't really be tested in the normal sense. You can make a theory and then try to look for more data/make observations about the universe and see if they match, but there's no control and you might never get the data you need to say one theory is better than another. You can get lucky of course, but it's not like you can recruit 40_000 universes for a double blinded placebo controlled clinical trial.
You can at least test some parts of quantum physics and general relativity on and around Earth, with satellites in orbit with very precise clocks and double slit experiments and that sort of thing. For everything outside the solar system you can just observe and hope new data arrives that you are not already unblinded to.
