Hindmost

In an alternate universe, Hindmost was the most remote significant piece of baryonic matter known to the Time Lords; it was so far from Gallifrey that a TARDIS would have to be specifically re-engineered to make the journey. Hindmost was described as being a planetoid barely big enough for a man to stand on. The planetoid was only detectable by the Time Lords since it was so far out of the expanding universe. (PROSE: The Infinity Doctors)