E-Space
E-Space, also known as Exo-Space, is a smaller universe existing alongside the prime universe (also known as N- or Normal-Space). It is not a parallel universe as that term is generally understood, having its own astronomy and unique inhabitants. E-Space is visually distinct from N-Space in that it possesses an unusual greenish tint.
The only known worlds in E-Space are Alzarius, the Unnamed Planet in E-Space which was the hiding place of the Great Vampire, and possibly Terradon, although it is also possible the starliner also originated in N-Space and entered E-Space by accident like the TARDIS.
The Doctor appeared to suggest that N-Space is a positive universe and E-Space its negative counterpart. This was based upon the fact that the TARDIS scanner continued to show images of N-Space even after materialising on Alzarius, and that the scanner system, rather than being a simple optical relay, apparently depicts whatever exists at the ship's current co-ordinates. The Doctor suggested that the system operated off the absolute value of the co-ordinates and thus could not process negative co-ordinates. The scanner system was made functional again simply by replacing its original N-Space image translator with one native to E-Space.
Travel between N-Space and E-Space is a hazardous and apparently random process, even using a TARDIS, requiring passage through a Charged Vacuum Emboitment or CVE. On his return journey to N-Space the Doctor passed through a strange white void known to its most frequent inhabitants, the Tharils, as the Gateway. (The Gateway zone between N-Space and E-Space had zero co-ordinates, thus adding credence to the 'positive and negative universes' theory.) It is not known whether it is possible to travel from E-Space to N-Space without physically visiting the Gateway.
The Logopolitans of N-Space revealed they were responsible for the CVEs linking E-Space to N-Space, as part of their plan to forestall the overdue entropic collapse of N-Space by opening up what had previously been a closed system and thus dissipating the accumulated entropy. Following the disastrous consequences of the Master's visit to Logopolis only one CVE remained open, from Terran perspectives in the constellation of Cassiopeia. Unless further CVEs were later created it is highly likely this is the route through which the Hydrax entered E-Space several centuries later.
