What are you absolutely sure about but can't prove?

For me, it's that Deja vu is a glitch in the universe. That it's proof that there may be another universe or universes out there.

Maybe I'm crazy but I've just always thought that with Deja vu even though I now know that Deja vu just might be a momentary malfunction in the brain's processing of sensory information.

But it's still fun to think that there might be multiple universes and deja vu might be the proof.
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I put this as other as a topic. Not religion & spirituality. Ugh.
What are you absolutely sure about but can't prove?
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