It should come as no surprise to anyone that the universe is dirty. No matter where you go, in space or in time, you can be guaranteed to encounter a certain amount of muck and filth; it’s something that can quite accurately be termed a universal truth, and yet humans always seem disappointed by it.
The Doctor’s current companion looked at their surroundings with a distasteful frown. “Somehow I always thought the future would be cleaner.”
“Dirt breeds,” the Doctor explained. “Wherever there are living organisms, there is bound to be dirt, because without dirt, there can be no life.”