But you, yourself, exist here, in the universe you exist in. Parallel universes are simply probabilities that could be where you exist, and often become where you exist for a very short time. Most of them you never exist in.
We are each part of our own universe(s). Like cryptography, even as there are an infinite number of combinations if the length of the hash is infinite, even so there are an infinite number of parallel universes. We each live in our own universe, except once in a while, where a soulmate might join you for certain times in your travels through the universes you travel through.
Understanding parallel universes is not a simple thing. In fact, regarding science reality of PUs, their impingement at points upon each other is what cause the existence of energy waves and subatomic particles. The form in which these subatomic particles exist, is expressed in the material they become. In other words, all matter is made up of countless contact points where various parallel universes momentarily touch each other.
The aether is simply the "flowing" of the parallel universes among each other so that they contact each other at various points and "times." This is entirely non-understandable, because it includes all the dimensions, as well.
Even if you recognize your travels through some of the parallel universes you pass through, you will never be able to explain it adequately enough that someone else will really understand.
