[Vecr]

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.

[literalAardvark]

That's poppycock. RCTs aren't necessary for high quality evidence, in neither physics, nor nutrition.

[Vecr]

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.