Edge of the universe

The edge of the universe, edge of reality, end of the world or edge of the world, which Astrolabus equated with "the ends of the Earth", was was the location where space actually ended. The Sixth Doctor claimed that "the world [had] no end", but was told by Astrolabus that this statement was a product of logic and that "logic [was] a new toy". Beyond the edge of the universe was the Void.

History
Mondas was said to have drifted to "the edge of space". (TV: The Tenth Planet)

The Empress of the Racnoss travelled to the edge of the universe in her Webstar, where she hibernated until the Secret Heart was uncovered. (TV: The Runaway Bride)

The Dæmon Azal travelled to the edge of the universe in the spacecraft Leviathan, where he encountered and merged with his counterpart from another reality. (COMIC: Voyage to the Edge of the Universe)

According to Salamar, Zeta Minor was located "on the very edge of the known universe." (TV: Planet of Evil)

The Fourth Doctor and Leela visited a planet on the edge of the known universe, where they fought vampires. (AUDIO: White Ghosts) They later travelled to the "edge of the cosmos", where they detected the R1C's ion drive and encountered a spiral nebula. (TV: Underworld)

The Fourth Doctor, Mike Yates and Fenella Wibbsey travelled to the edge of the universe, where the Demon had create the Sepulchre atop a planetoid. (AUDIO: Sepulchre)

In 1999, Anne Travers banished the Great Intelligence, stranding it on the edges of the universe, riding the blue shift outwards into infinity. (PROSE: Millennial Rites)

Astrolabus was hounded by Voyager to the very edge of the universe, a frozen wasteland with some rocky formations and a stormy sea which streamed from the edge downwards and into nothingness. He landed his TARDIS in the form of a lighthouse and use it to lure ships to their doom. He successfully found the ship of the humanoids who had taken his charts from him, only to be delayed in departing by the Sixth Doctor. It was with Voyager, the rightful owner of the charts, hot on his heels that Astrolabus left the edge of the world. After telling the Doctor to find Astrolabus on his behalf, Voyager sailed his death-ship off the edge of the universe and into the Void. (COMIC: Voyager)

fled from the Krotons to the edge of the universe, where he collected the exotic particles he needed for use against the Chronovores. (PROSE: The Quantum Archangel)

The Monk travelled to the edge of the universe, where he summoned Artemis. (PROSE: No Future)

The Eighth Doctor and Molly O'Sullivan travelled to the edge of the universe, where they encountered Liv Chenka and the Eminence. (AUDIO: Time's Horizon)

The Thirteenth Doctor visited the Unmarked Purlieus, an uncharted star system at "the very edge of reality", where she was infected by a psychic virus. (AUDIO: Salvation)