Entropy
From Tardis Wiki, the free Doctor Who reference
Entropy, represented by the second law of thermodynamics is the slow, irreversible process of physical decay.
Because of this, N-Space, the main universe, had a finite lifespan and would have "died" previous to the 20th century if the Logopolitans had not siphoned entropy into E-Space by means of CVE's which they created using block transfer computations (DW: Logopolis, NA: Timewyrm: Apocalypse)
In N-Space and in E-Space, the entropy appeared as a green tint seen both in the sky of planets and in space itself. (DW: Full Circle, State of Decay) In 1981, the Master interfered with the Logopolitans and closed the CVE, a wave of entropy destroyed most of the universe. (DW: Logopolis)
