Thank you, @jayna!
I have a theory, or rather an idea, but I once thought that what if deja vu is just a result of living a moment in real life that we might've experienced in a dream, but we just can't remember (consciously) we experienced it there.
Not sure if we can time travel in dreams and such, but they're fun things to tinker with at least.
Yes, at the end of the day, how do we even know what’s possible, and whether the reality we are conscious of is the only reality. I like your idea about deja vu.