Black Void

The Black Void, also known as the Great Void or (in its humanoid aspect) by the nickname of "Goldilocks", was a "howling emptiness" existing outside of the universe, at the "very edge of Time and Space". It was "the canvas on which reality was painted". It was also "a place where things that didn’t exist howled and raged against their nothingness". (PROSE: Past, Present & Yet To Come)

Nature
The Black Void was the void that existed before the Big Bang and was being filled out by it, space without any kind of matter or energy. On the edge of the universe one could see quasars and other forms of cosmological expansion happening. (PROSE: Fear of the Dark) the Dark existed in the void before the Big Bang. (PROSE: Fear of the Dark)

The Black Void also existed once again at the end of the universe as its natural endpoint. Positioned just before the end of Time itself was a liminal span of existence called the Boneyard, overlooking the Black Void, which was a place where the heroes of the universe came to die peacefully.

The Black Void was actually sentient and tried to enter the universe and "optimise" it to its needs, constantly rewriting the timeline of its physical embodiment to optimise itself to shifting conditions, and "optimising" the timelines of other individuals. It was ultimately tricked and defeated by the Crone, Lucy Wilson's distant future self, who cast it back out of the world at the very end of the universe; due to its own timeless nature, however, this was also the very banishment that had placed the Black Void at the beginning of the universe in the first place. (PROSE: Past, Present & Yet To Come)

History
The Black Void was the void that existed before the Big Bang. Over the course of Time, the universe slowly expanded into that void. (PROSE: Fear of the Dark)

After arriving in the White Void located "outside" of the Land of Fiction, which the Second Doctor stated was "nowhere" and outside of time and space, an intelligence believed to be the Master of the Land attacked the Doctor's mind as well as his companions Jamie McCrimmon and Zoe Heriot. The TARDIS relocated to the black void, where it turned over and over before losing its dimensional transcendence and breaking apart as six sides of a police box. The Doctor drifted away from Jamie and Zoe, who held onto the console before being engulfed in smoke. They eventually arrived in the Land of Fiction. After defeating the Master Brain and causing the Land of Fiction to dissolve, the TARDIS travellers and the Master faded back into the black void where the TARDIS reassembled itself. (TV: The Mind Robber)

The Monk summoned Artemis the Chronovore at the edge of the black void. (PROSE: No Future) Additionally, the Dark existed in the void before the Big Bang. (PROSE: Fear of the Dark)

The Watcher took the Doctor's TARDIS out of space and time, from which Adric and Nyssa would be safe and could observe the entropy wave had unleashed following the destruction of Logopolis. (TV: Logopolis) The demon Melanicus was trapped in the black, formless void for a thousand years until he escaped through the Event Synthesizer. (COMIC: The Tides of Time)

The Black Void attempted to enter the universe, taking on the appearance of a two-dimensional shadow creature with a wide mouth and scissor-like fingers. Unable to quite fit inside reality, it existed in the Interface, a liminal state of existence from which it kept "optimising" its own timeline, rewriting the nature of its spaceship to adapt to changing situations. It attempted to "optimise" the entire universe, bit by bit, until it could exist inside it, but was stumped by fixed points in time such as Lucy Wilson.

It tortured an adult Lucy for 300 years with images of Hobo Kostinen's potential deaths until she agreed to join its crusade as "the Crone". However, Lucy had actually tricked the creature, whom she had nicknamed "Goldilocks": after luring it to the Boneyard, she expelled it from the universe and sealed it out. Due to its timelessness, this was not only the Black Void's banishment at the end of the universe, but also the reason it existed removed from the universe at the beginning of time, as well. (PROSE: Past, Present & Yet To Come)

Behind the scenes
The relationship between the Black Void surrounding the physical universe and the Void (the space between "universes", plural) is unclear, no two stories having ever used the two in conjunction. Although at first glance the two might appear to be two names for the same concept, and certainly the two Voids are applications of the same literary tropes, the Black Void appears to merely be the empty space all around the physical universe, whereas the Void Between Worlds is described by the Tenth Doctor in TV: Army of Ghosts as containing "absolutely nothing", including "no up, no down (…) no space, no time".

Past, Present & Yet To Come added further mystery to the Black Void by writing it in such a way that it can be read as an identity of the Enemy. Certainly more of a process that an individual species, the Black Void is depicted as a force that is working to rewrite history to its own designs in order to better fit itself into the universe.