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)

A doorway to this pocket universe in the form of a crack in time appeared in the town of Christmas on Trenzalore. It was through there that the Time Lords sent the message "Doctor Who", which attracted the Eleventh Doctor as well as various races across the universe, and, 900 years later, with the Doctor dying from old age at the end of his final incarnation, energy for a new regeneration cycle. (TV: The Time of the Doctor)

The Time Lords were eventually able to return Gallifrey to their original universe, 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 Timeless Child, which the Time Lords had kept from him and the Doctor. (TV: Spyfall)