What do you believe deja vu to be?

theres plenty of well documented theories as to what it is

you may enjoy reading some of those

personally I feel that the theory which makes most sense is the glitch between memory lodgement and matching is most logical

when we perceive something in the world, we instantaneously process and search out matches and then commit the whole thing to memory

so, we see something new, we process what we are seeing, we seek to make a match from memory which will aid in our processing and then filing the new memory

what if on this occasion theres a glitch whereby what we are perceiving doesnt get processed in the normal way rather is immediately filed in the memory then the continued experience is processed and a memory match is sought - now we find a match - voila, "Ive experienced this before"

I think it is the same as dreams we have.

Our subconscious mind takes us where we wish to be.

We see many movies,many books and hear almost infinite words throughout our life.This makes us think we experienced it before.

Studies have been done with many theories but no concrete answer.