Temporal exclusion zone

Observed by most time sensitive species, a temporal exclusion zone covered the Dark Times of the early universe. During the Last Great Time War, the Time Lords' scrutiny of Dalek history brought forth a range of sources providing "recent intelligence" that indicated the Dalek Time Squad had somehow managed to enter the Dark Times, at the periphery of the temporal exclusion zone. (PROSE: Dalek Combat Training Manual)

Even during the Darkest Days of the Time War, it remained impossible for the conflict to enter the Dark Times of the universe, with no one managing to break through the barrier surrounding those earliest days of the universe. (PROSE: Secrets of Time Lord Victorious)