Ever experienced going through something you thought has happened previously? Or have you ever asked someone why he or she is so familiar although you've never met before?

Many people have probably experienced déjà vu. The French term which literally means "already seen" has always been a mystery for years --- challenging scientists to come up with a specific explanation.

Dr. Akira O'Connor, a researcher specializing in memory at University of Andrews said Psychologist Alan Brown described the experience as "a subjective experience of familiarity, alongside an objective experience of unfamiliarity."

"That is, you know something shouldn't feel as familiar to you as it currently feels," O'Connor added. 

"There are a bunch of different ideas about what might cause déjà vu but the jury is very much still out on a definitive answer," O'Connor wrote in an email to Buzz Feed Science.

According to him, the best theory so far is that déjà vu is caused by alterations in your brain chemistry. "The basic idea is that déjà vu results from a mini-seizure, like those experienced by people with epilepsy, but without it generalising to the rest of the brain and having catastrophic effects," he said. "It's like a tiny excitatory twitch in the memory-region of your brain that doesn't go on to affect anything else."

Furthermore, evidences have shown that the brain chemical, dopamine may also be involved.

Dopamine is a neurotransmitter that sends signals from the brain throughout the body.

According to a study published in the Journal of Clinical Neuroscience, medications affecting the dopamine activity have also triggered déjà vu. This was shown in the case of a patient who was constantly experiencing déjà vus when he was taking a couple of flu medications.

"For what it's worth, their patient completed the full course of medication because he enjoyed the experience so much," O'Connor said.

The patient stopped experiencing déjà vu when he was done with his prescription.