Parallel universes exist by virtue of inflation in space or the movement of space which is essentially a fluid. It is the distance traveled by the multiverse at the time when the time itself is slower than in the reference universe in which we live. What causes the space to move is the absence of matter around it, other than the space that has been producing gravity which is still static in nature and could be assumed to be still. So we have space that is moving besides the space that is bound around the matter. The whole matter of the universe attracts the space thus causing it to ensemble towards a point direction around the matter itself. But the space that is moving is receding away from the point of creation,Big Bang, primarily because of absence of matter and since then has been having inflation.
Under such conditions the space that is having slower time exhibits parallel universe and could be run into it if the observer himself is having slower time and moving at certain critical speed. The other scenario could be when the moving space reaches the observer by virtue of its movement and the observer is unable to enter the multiverse because of lack of critical speed.
Still the observer experiencing slower time might only see the world through psychological time that affects consciousness.
The moving space produces multiverse which are separated on the basis of number of dimensions or the slowness of time.