Unspace
Transwarp is the name for the general nothingness that lies in the for-purposes-of-description-let's-call-it-"space" between realities. Space bridges, Time travel, subspace storage pockets, Transwarp Drive, and most other forms of faster-than-light propulsion all involve transwarp somehow, even if their builders don't always quite understand the specifics. The huge amount of energy needed to connect two discrete points in a single universe, or the utterly immense amount it takes to go against the natural flow of time makes travel through it slightly, very slightly, a little ...unreliable, as you might not even end up in the same time or universe you were starting from. Or aiming for. Or even in a universe at all. Fortunately for frequent travelers, the inhabitants of one of the older, more advanced realities have kindly set up a safety net of sorts, oftentimes rescuing those, both the on-purpose and the accidental, otherwise lost to the not-always tender mercies of the multiverse.
For an entire non-existence of formless nothing, it's got some who call it home. You probably wouldn't want them to notice you.
