The entire universe has a 5th dimension called the mirror universe. It is layered on to the physical universe so that spirits and memories are all around us. This is also where the pasts and futures are.

It is also why people think that aliens and MIB are extra-dimensional beings. MIBs are not. They are time travelling by entering this mirror universe and entering our physical time-line from there.

Nobody really understands what an extra-dimension is. They know the term, but don't get it.

There is only one relevant invisible dimension meshed with ours. It is the mirror dimension. "The Other World" as you say.

Everyone living and dead has a mirror body in that 5th dimension. That is where your soul is.