Before the Big Bang, there existed the pre-universe. Accounts differ as to what it actually was.
According to most accounts, there was another universe before the Big Bang which operated on different laws of physics. (TV: The Satan Pit, AUDIO: Persuasion, PROSE: Millenial Rites) However, the Carnival Queen indicated that this was not a separate universe, but the state of reality before the Time Lords created the Web of Time and, by the Doctor's time, those who knew about the pre-universe logically assumed that it was the previous universe. (PROSE: Christmas on a Rational Planet)
The Beast, who purportedly came from this time, stated his origins were from the time before N-Space "came into existence" and before "the cataclysm". The Tenth Doctor disbelieved this, saying, "No life could have existed back then." (TV: The Satan Pit)
When N-Space was created, many powerful species fled into other realities and over time many found their way back into N-Space. Some (such as the Great Old Ones) still followed the rules of the pre-universe, giving them god-like powers. (PROSE: All-Consuming Fire, Millennial Rites, Christmas on a Rational Planet)
The Doctor described the pre-universe to Anne Travers.
Before this universe was created, there was another one. A totally different universe, with alien physical laws. The heavens were green, and the stars looked like... giant doughnuts, to be brutally frank. Very, very different. And, as in this universe, there were people who discovered the deeper mysteries of time and space.
Because of new laws like the speed of light or the charge of the electron, the Great Old Ones found they had great power and became terrible gods. (PROSE: All-Consuming Fire) The Shepherd and Shepherdess escaped the "extinction of their universe" and arrived in N-space where their appearance became transparent. They moved without touching the ground and their hair and nails grew longer because they were dying in the new reality. (AUDIO: Persuasion)
Amongst the races that claimed to be older than the universe itself were the Great Old Ones such as Azathoth, Yog-Sothoth and Shub-Niggurath. (PROSE: All-Consuming Fire, Millennial Rites) On many planets, these beings became part of the legends and religions. The Great Old Ones were worshipped by the first intelligent species to evolve on Earth - the Silurians. (PROSE: All-Consuming Fire)
The Beast claimed to have originated before the universe. He said was imprisoned by the Disciples of the Light deep beneath Krop Tor "before time... before time and light and space and matter, before the cataclysm, before this universe came into existence", likely placing both in that time. The Tenth Doctor believed that "no life could have existed back then." The Doctor wondered if the idea for all Satan-like figures in the mythology of "a million worlds", including that of Earth, Draconia, Vel Consadine, Dæmos, and the Kaleds' God of War, had maybe bled through as a thought in the back of "every sentient mind". The Beast himself claimed to be the Devil of "all" religions, which included the Arkiphets, Quoldonity, Christianity, Pash Pash, New Judaism, San Klah and the Church of the Tin Vagabond. (TV: The Satan Pit)
Knowledge of the pre-universe was a closely guarded secret. The Sixth Doctor only knew about it as a result of his connection to the Matrix during his time as Lord President. (PROSE: Millennial Rites) During his encounter with the Carnival Queen, the Seventh Doctor learned that the Time Lords had misinterpreted their knowledge of the pre-universe. (PROSE: Christmas on a Rational Planet)