Edge of the universe

The edge of the universe was the location where space actually ended, beyond which there was nothing but a Black Void.

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 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)

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)

Behind the scenes

 * In the real world, the universe doesn't have an edge because space is curved. However it is possible that the term "edge" isn't meant literally, and just refers to the outer-most part of the universe, right beside another universe.