We can't build a time machine ever, and there's plenty of evidence to support the fact that it's a meteor.
I suppose it could be me going back in time and exhaling badly...

But uh, if you want to get technical, according to what you said, everything is a theory. Please take no offense if that's not what you meant.
Including that we discovered fire, because there's no proof that's lasted throughout the last however many dozen thousand years it actually happened.

Unless you take cave paintings as proof.
So, why do you say that it's just a theory? Because really, without a time machine, we can't prove we existed before Ancient Sumeria.

We humans have been around for 2 million years, you're telling me that's a theory. Did we just suddenly appear? Are you saying we need a time machine to prove the Earth was created due to gravity compressing mountain sized chunks of rock together over and over again? Or to prove the creation of the sun? How about the big bang?
TBH, a time machine is a cop out. You can't use it, because it is impossible with what we know.