Highest Entropy Matter
From Tardis Wiki, the free Doctor Who reference
Highest entropy matter, commonly abbreviated as HEM, was a type of matter that required vast amounts of energy input for basic particle interaction to even begin. As a result it was used in the War to inhibit energy transfer. It was harvested from the end of the universe by nechronomancers. (PROSE: The Book of the War) After the Cold was used on Ordifica, it was covered by a black substance to stop the creatures from escaping. (PROSE: Interference)
