In my opinion, we need to do much more work to figure that one out. Many people have been trying since way before I was born to marry general relativity and quantum mechanics, and there’s no clear way to do that yet.
There are no theories that prove its existence because that’s not something theories do. It’s the job of experiments to figure out if something exists or not. On the flip side, theories figure out if something _could_ exist to explain [thing we don’t understand], given [thing we do understand]. So there’s this back-and-forth between theory and experiment through which one grows as a result of growth of the other. But to come back to my opinion, we need a fair share of growth before getting to the graviton.