Gallifrey's pocket universe
At the end of the Last Great Time War, the planet Gallifrey was relocated from N-Space to a parallel pocket universe, (TV: The Day of the Doctor) which the Spy Master described as its little bubble universe. (TV: Spyfall)
History
In order to save the planet from the Daleks, "all thirteen" incarnations of the Doctor teamed up together to freeze Gallifrey in time and place it in a "single moment in time, held in a parallel pocket universe." While they were unsure if they succeeded, an encounter with the Curator indicated to the Eleventh Doctor that this was in fact successful. According to the General and the Doctors, the calculations necessary would take several hundred years so the Doctor worked on them for all of his various lives. (TV: The Day of the Doctor)
Though readily able to leave the pocket universe, the Time Lords were unable to locate N-Space. When the Matrix detected a crack in time in a nearby universe, the Time Lords took control of it, (PROSE: A Brief History of Time Lords) and began transmitting "Doctor Who" through it, hoping that the Doctor would answer with his true name and confirm that it was safe to leave the pocket universe. The message was sent out from the town of Christmas on Trenzalore. The Eleventh Doctor and all the races of the universe heard this call and fear of the Time Lords caused the nine century long Siege of Trenzalore. At the end of the conflict, the Doctor, having expended all twelve of his regenerations, was dying of old age. The Doctor's companion, Clara Oswald spoke to the Time Lords directly and beseeched them for aid which they granted by granting him more regenerations. (TV: The Time of the Doctor)
With the events of the Siege confirming that they'd found N-Space, the Time Lords returned to it, (PROSE: A Brief History of Time Lords) through a manner the Doctor purposefully didn't try to find out, in order to not make the other Time Lords "feel clever". In doing so, the Time Lords displaced Gallifrey in time, and it now existed at "the end of the universe". This was ultimately discovered by the Twelfth Doctor. (TV: Heaven Sent, Hell Bent)
The Master, however, recalled to the Thirteenth Doctor that Gallifrey was still "hiding in its little bubble universe" when he returned and ravaged the planet in retaliation for the secret of the Timeless Child, which the Time Lords had kept from him and the Doctor. (TV: Spyfall)
