[neutralino]

I don't think this poll can be answered, at least not without defining some terms. Firstly, we need to define what we mean by "universe." Do we mean everything that exists, or do we mean everything that exists in our 4D spacetime mainfold? There are braneworld models that model our universe as a 3D brane moving in a 6D space. Other universes are then other 3D branes moving in the "bulk." [Someone mentioned this model above, but incorrectly said that other galaxies were the other universes: not true.]

Ok, so now we presume we have a definition of universe and, thus, a definition of "another universe." The first answer to the poll seems the obvious one "there are no others" and in fact this will necessarily be the answer if we define the universe to be everything that exists. I would immediately throw out the second option, unless there is a new use for the term "void." A void is, basically, a patch of empty space. However this is empty space and thus, by definition must be in our universe. Any other matter past this must also be in our universe, since there must be some boundary between the void and this other matter.

Option 3 is the one used by sci-fi writers all the time. But what does parallel actually mean? I would seek some clarification on that before selecting that as an option. Option 4 seems to be a reason answer.

To summarise, obviously I don't know the answer, but I stress that we first define what we mean by terms before we can answer this, and any, question.