Depends on which theory you prefer. A parallel universe isn't going to be looking back down at you... that'd have to be a mirror universe
. There area lot of fictional suggestions and quite a few theoretical scientific suggestions ranging from bubble universes to multiverses. I quite like like the idea of a 'quantum multiverse' where all aspects are the tiniest quanta of all the options and our consciousness acknowledges a small subset of these which make up our own experience. That means that in some iterations of assembled quanta I'm having it rough but in at least one possible arrangement I'm somehow surrounded by attractive young naked ladies impressed by my huge penis and sexual prowess.......
There are some great scifi ideas.. from James Blish's 'Utopia' where anyone can ask to me mayor... the office comes with an irremoveable chain of office and there are voting booths everywhere. If the ayor upsets too many folk who vote against them then that chain explodes and another citizen is elected at random. Politicians there tend to be careful with their rules and laws. there are also no police on Utopia so folk make sure they make friedns with lots of people so if anyone harms you then your mates take their revenge.
There was another story about the world Jones made (can't remember title and author) where everything of existence was within the mind of Jones. Within this Jones is the only one who doesn't know he created it and when he starts to have self-doubt then the universe begins to shrink and disappear.
It's well worth a punt about the internet for the more scientific arguments about these if into sci-fi
Whatever way you argue that is different to the huge size of our known universe, the arguments for life elsewhere and the problems of travelling to other star systems. A neat argument to support faster than light travel i came across recently was a suggestion that mass is an interaction with subatomic particles and something akin to dark matter - the 'zero point energy' that residual vibrational energy at absoulte zero temp. The theory suggested that if one could harness that energy and use it then mass would reduce...and allow FTL because the faster you fly using that surrounding energy then the more you access and the less mass you have - which allows one to break Einstein's limit of C without breaking his formula.