Space Time Universe

One "Space Time Universe", named such by the second Dr. Who, was visited by him and his companions Jamie and Zoe. Due to the laws of nature differing in this universe, gravity did not exist. The planet they visited was home to many large metal robots who were abandoned by their creators. (PROSE: A Thousand & One Doors)

History
This universe was sealed off from the universe familiar to Dr. Who, but a "door" (a tangential point) between the two broke through several times, causing oxygen to leak through. The Tardis came through this door and landed on the planet, where the travelers began floating due to the lack of gravity and were abducted by mysterious beings known as Tabro and Belunda. They brought them to the head of one of the giant handling robots abandoned, and began studying them due to their unfamiliar biology.

Surprised that Dr. Who could speak, he was told that they were dimension travelers from the Eight Thousand and Ninety First Universe, who had found a thousand such "doors" connecting different Space-Time Continuums. Due new doors opening connecting the Tardis travelers' Five Hundred and Third Universe with their home via the unnamed Space Time Universe, poisonous oxygen impinged on their home and Life Force. They intended to destroy the oxygen-breathers and the planet before sealing the doors, but Dr. Who and Jamie disarmed them and the crew returned to the Tardis. (PROSE: A Thousand & One Doors)

Behind the scenes

 * The "Space Time Universe" is never given a numerical name by the Life Force's explorers; instead, it is described as such when the Doctor leaves, saying: ""Now we have indisputable proof that this is a multi-dimensional Cosmos, with probably many tangential points where one could pass from one Space Time Universe into another. But... This is no Space Time for us.""

- Dr. Who Within the story, terms like "universe", "dimension", and "Space-Time Continuum" are conflated as synonymous, whereas later stories might not treat them this way.
 * This Space Time Universe is also one of, if not the first example of a parallel universe seen in Doctor Who media. The concept would make its way onto television in the next year's Third Doctor story Inferno.