Maybe not definitive proof, but interacting with other agents in your universe (I.E the rest of us) and sharing experiences could prove that the universe is bigger than just you and the words on your screen.

Then you slowly start to realise that all of this activity on Earth could be happening on other planets elsewhere too.

Then you observe the stars, and combined with the mental models/theories from our brightest physicists, you might conclude that the universe is really fucking big, and there must be a shitload of other Earths out there.

But again, we can't really prove it concretely can we? For all you know, i'm an NPC and so is everyone else. You could the subject of an alien crackintro - where they try to simulate whole worlds in 100 bits or less.