Entropy

Entropy, a product of the second law of thermodynamics, is the slow, irreversible process of breakdown and decay whereby a complex, energetic system inevitably degrades into a simpler, more static one.

Being a complex and energetic closed system, N-Space, the main universe, had a finite lifespan and would have "died" some time prior to the 20th century had the Logopolitans not vented entropy from it into E-Space, another universe, via the CVEs which they created (DW: Logopolis, NA: Timewyrm: Apocalypse)

The stellar void of E-Space appeared to have a green tint, seen both in the sky of planets and in space itself, which may have been due to the accumulated entropy. (DW: Full Circle, State of Decay)

In 1981, the Master interfered with the Logopolitan project and inadvertantly closed the CVEs. A wave of entropy destroyed a large portion of the universe. On the homeworld of the Logopolitans, machines affected by entropy ceased to function and then crumbled. Parts of the Monitor, the Logopolitan leader, disappeared unevenly immediately prior to his actual disintegration. Whole stars and galaxies and many regions of the universe vanished, such as the Traken Union. (DW: Logopolis)