Void

The Seventh Doctor once joked that the first law of space-time travel was to "avoid voids". (PROSE: The Highest Science)

The Thirteenth Doctor determined that the Kasaavin realm was "not really a void". (TV: Spyfall)

According to one account, which admitted to not knowing the full story of the conflict, battles in the Last Great Time War were fought not in physical space, but in the Time Vortex and "even beyond", in the Ultimate Void. For this reason, most of the lesser species knew nothing of the War. (PROSE: Meet the Doctor)

The Tenth Doctor identified the Void as "the space between dimensions". (TV: Army of Ghosts)