What convinced you the universe alone is all that exists?

[Ophiolite]

Your feelings?

Dogmatic assumption? <-- Whether yours, or someone else's?

Belief in the myth of Conflict Thesis?

Something else?

Or, is the conclusion part of a larger step-by-step process that you can explain in clear detail without being painfully vague and ambiguous?

I'm not convinced that the universe is all that exists. I can see no sound reason for thinking this is the case and little reason to suppose it isn't. The question is beyond our pay grade and beyond our current ability to determine, and therefore isn't worth spending much time on, other than over casual chats with friends in a bar, or acquaintances on the internet.