Existence of Parallel Universes

I know a barely anything about parallel universes, but my ignorance has caused me to raise the pregunta as to how a parallel universe could exist in el espacio around us.
Assuming all that surrounds the physical boundaries of our universe is absolute nothing with no properties whatsoever, any universe existing in that area outside our own would have 0 distance between itself and our universe due to the fact that absolute nothing has no length/width/volume/whatever else to separate two universes.
I've heard the argument raised that what is outside the universe is not absolute nothing but is full of quantum particles, but I'm fairly certain we cannot assume that the laws of quantum mechanics that persist in our universe would not exist and/or have the same rules and particles outside our universe. Additionally, that begs the pregunta as to how these particles got there in the first place.
Can anyone clarify to me how science justifies the existence of parallel universes? I can't seem to find any notes on the subject which I can adequately process dado my limited knowledge on the subject.