so, I'm going with the general sci-fi principle that lightspeed travel is possible via hyperspace, the most useful of imaginary scientific concepts. I'll admit to not having thought about hyperspace much, 1. because star wars did it for me and 2. because its not at all significant to the plot, wheras magic, time-travel and void-travel are, so I put effort into developing detailed systems for those instead. (which I love talking about
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yes, there is an inter-universal void space, which essentially acts like normal space. It takes time to cross in a void ship, it has vast gaseous nebulae as a result of matter leaking through from universes. All universes exist within it, and appear as vast, silvery spheres.
The proximity of universes to one another is based on how frequently their timelines diverged, splitting them from one universe into two or more. Universes that are close together and split recently will be fairly similar, while ones further away will be vastly different, having separated billions of years ago.
It would take maybe a a couple of months to travel from one side of our local cluster of universes to the other, and about half an hour to travel to a universe very nearby.
It is also possible to move directly between universes without going into the void, if a rift opens; but inter-universe rifts are so rare that most time-travellers and void-travellers believe they are pure myth.