Following the fall of Gallifrey, the events of the Last Great Time War between the Daleks and the Time Lords were contained by a time lock, thus isolating the conflict in its own specific timeline", (PROSE: Dalek: The Astounding Untold History of the Greatest Enemies of the Universe) which was mostly separate from the normal universe. (PROSE: Revenge of the Nestene)
According to conflicting accounts, the time lock was placed by either the surviving higher species (PROSE: Dalek: The Astounding Untold History of the Greatest Enemies of the Universe) or the Doctor. (COMIC: Don't Step on the Grass) This time lock was intended to ensure that no entity or faction could use time travel technology to revisit the war to alter or otherwise engage with any of the events that had occurred. Such interference was deemed too dangerous, likely to place the integrity of the Time Vortex itself at risk. (PROSE: Dalek: The Astounding Untold History of the Greatest Enemies of the Universe)
At the end of the Time War the time lock tore right through the planet Lujhimene, severing it in half. The time lock maintained the planet's gravity and atmosphere, and also spun its core as if nothing had happened. The Eleventh Doctor visited Lujhimene looking for the Volatix Cabal. (COMIC: Running to Stay Still)