A Mobile Universe is a Universe constructed specifically for quick mobile travel.
Usually, a Mobile Universe looks empty, only as an outer shell, like a Hollowverse, but in reality, it is being used to its fullest extent by a very highly advanced civilization, that is utilizing it for whatever reason necessary.
These Mobile Universes can travel anywhere from extremely slow finite speeds, to unimaginabely fast infinite speeds. They also only really exist in the billion-light-year size range, because Mobile Universes only serve very specific functions and are rarely used. If a transportation device is needed and the type of transport does not require a Mobile Universe, it won't be used. If the ship needs to be smaller than 23 Bly or larger than 203 Bly, a Mobile Universe won't be used. The Mobile Universe does what it does best at that size range, but there are more effective methods available.