There are multiple existences, parallel universes, if you will. A myriad of them. I think déja vu is just when your life in this existence and another get so similar to each other, that eventually, they merge for just a second or two, then go their seperate ways again, and this gives you a strange sense of... Well, déja vu.
I made this picture to show how it works:
Another theory, one more commonly believed, goes like this:
There is a slight delay between you seeing it and your brain registering you've seen it. I don't really get it, but that's what my dad says, lol.