A Universe consists of all existing matter and space considered as a whole, known as the cosmos. The universe is believed to be at least 10 billion light years in diameter and contains a vast number of galaxies; it has been expanding since its creation in the Big Bang about 13 billion years ago.
The Original Doctor Who Universe was overseen by the Time lords who allowed access to other universes but this was apparently stopped by their destruction in the Time War. The inhabitants of Logopolis were the first to gain access to other universes through CVE's which they opened to prevent the universe's heat death. But in 1981, the Tremas incarnation of the Master interfered with operations on Logopolis causing Entropy to wipe out a mass majority of the universe, creating a gaping gap in the structure of the universe. Several galaxies were destroyed and only the opening of a CVE in the constellation of Cassiopeia prevented the universe's total annihilation.
Following the Entropy incident, the Timelords adopted the Logopolitians method of Charged Vacuum Emboitment to create gateways to other universes. They intended to create a new clean universe to avoid a total timeline collapse on the universal spectrum of N-Space.