Deja vu stands for "ALREADY SEEN". There are researches going on this. I just thought to give an experimenting tool to people who experience it frequently.
All of us at some point in life must have experienced deja vu.Some could have experienced it once or twice in life time, or some could have experienced it very frequently.There are many theories hopping around related to this phenomena.Medical science has tried to explain this phenomena as a result of memory anomaly.According to medical science short-term memories are stored as long-term memories and we feel that particular phenomena had happened much time before.There are concepts of parallel world.
The very first thing that appears in our mind is the event that we experience as already seen
"WHERE WE HAVE SEEN IT".It is very unlikely that same event will occur twice or multiple times in this real world.The only place which is left is our dream.Now I am using one unexplained event 'DREAM' to explain another unexplained event 'DEJA VU'.I believe that if we have to understand 'DEJA VU',we will have to understand 'DREAM' first but we will discuss only 'DEJA VU' here and definition of dream will be used as a theorem to explain it.
Dream can be defined as a journey without senses.We do this journey ,many of us remember this journey, many forget it.Our mind travel through time but due to its limited capacity it's difficult to recollect or remember the journey.We visit many places while we dream and those whose mind keep these information give them a sense of this journey which we term as 'DEJA VU'.
To realise this journey without senses, we can do some practicals.We can keep a record of dreams which we can remember and whenever we experience DEJA VU,we can check this with our record.This is not a proven way but we can try it and hope some positive results.