There are many Parallel universes or as I like to call them Time-Lines that occupy the same position as ours. Why I don't like using the term Parallel universe is because I think that this universe we occupy extends forever. Even though we can only see a small portion of it at the present time, it is not limited by our perception of it. If one keeps moving outwards, one will keep doing so for eternity even if one were to travel at the speed of light, one will never come back to the position from whence one started.
What I refer to as parallel universes are the many layers of other universes sandwiched within our own space. As an example, imagine the position you are currently existing in, wherever you are right now reading this document. Now imagine that if you could move to another parallel universe, this very position you occupy might correspond to a garden, or in another universe, intergalactic space, or in another universe inside a deep ocean and so forth.
We can call these other universes timelines because they exist side by side with our universe. Many people say that we are duplicated in all other universes but I do not agree with this point of view. In my opinion it would be an irrelevant waste of space to duplicate a program instance everywhere when the combinations of programs are infinite. I think a more efficient option is one program instance per universe.
My idea of parallel universe coincides with the vision in the movie Tomorrowland and I know that sometime in the future we will be able to tear beyond this veil and enter other parallel universe as easily as we move between different locations in our own universe.