Have you ever had Déjà vu, the feeling that you've experienced what you're currently feeling, before?

Have you ever had Déjà vu, the feeling that you've experienced what you're currently feeling, before?

Jokes aside, Déjà vu (which means "already seen" in French) is a phenomenon that approximately two-thirds of the population has experienced. And no one knows exactly why, though science has provided us with a few theories. Here are 5 of them.

1. Drugs

But we're not talking about an acid trip. One study, posted in the Journal of Clinical Neuroscience, found that a 39-year-old male experienced severe déjà vu after taking a combination of amantadine and phenylpropanolamine when being treated for the flu. Aside from the influenza, he was an otherwise healthy man. Once he stopped taking the medications, so did the intense déjà vu feelings, the study reported.

2. Your brain is taking a "short cut."

This theory, written about in Psychology Today, posits that your brain is trying to help you out when you're in a new situation by taking a "short cut" right to your long-term memory. They describe it as a "fleeting malfunction" between your long and short-term circuits in your brain.

It's like when your phone autocorrects something to a word you frequently use, except you didn't actually want to use that word.

When your brain skips straight to storing things in long-term memory, that's when you get the feeling of already doing something or already being somewhere before.

3. Your dreams are creeping into your waking life.

Psychology Today points out that many people feel a sense of familiarity in dreams, even in unfamiliar situations and surroundings. For example, you know that coworker who comes up to you and says something like, "Man, I had the weirdest dream last night, I was hanging out with Tom Cruise, but it didn't look at all like Tom Cruise, but somehow I knew it was Tom Cruise, and we were at my uncle's new house, but I haven't even been to my uncle's new house, but somehow, I knew it was my uncle's house, and..." and you're just like,

on the inside? Anyway, it's that dreamlike feeling of familiarity that also lends itself to the feelings we have when we have déjà vu.

4. Your rhinal cortex is acting up.

The hell's a rhinal cortex? It's an area in your brain that's sort of associated with memory. Some scientists refer to it as the "gatekeeper of the declarative memory system." Its main function is to detect familiarity in your surroundings. The theory that links rhinal cortex with déjà vu is that when you experience déjà vu, the part of your brain that accesses memory, the hippocampus, isn't activated, but the rhinal cortex is. Why? We don't know. But it would explain why we feel something is familiar without actually having a full-fledged memory of it.

Because no one really knows when someone is going to get déjà vu, it makes studying it a little unpredictable. But scientists found that those with epilepsy experienced déjà vu right before they had seizures, which made epileptics good déjà vu subjects in studies. In one such study, researchers were able to induce déjà vu in their subjects through the rhinal cortex and not through the hippocampus.

5. We're in a parallel universe.

Well, sort of. In a video for Big Think — titled "What Is Déjà Vu?" — theoretical physicist Michio Kaku talked about how déjà vu is most likely a form of memory glitch, but says it raises the question, "Is it ever possible, on any scale, to perhaps flip between different universes? And the answer there is actually rather unclear. We physicists believe, for example, that there is really a multiverse that exists even within inside our living room."

He brought up an example by Steven Weinberg, theoretical physicist and Noble Prize winner, of listening to a certain radio station, like BBC radio, in your living room. The radio is tuned into a certain frequency. But there are other frequencies in your living room like Radio Cuba, Top 40 rock stations, Radio Moscow — but the radio is only tuned into BBC radio. And that's comparative to quantum physics.

"We consist of atoms," Dr. Kaku explains. "Our atoms vibrate. But they no longer vibrate in unison with other universes. So in other words, déjà vu is probably simply a fragment of our brain eliciting memories and fragments of previous situations. However, in quantum physics, they really are in some sense parallel universes surrounding us. The problem is, we can't enter them... We're no longer vibrating in unison with them."

We're just not tuned into the frequency, but we know it's around us. Physicists believe that could be another source of déjà vu.