Darks are people who are considered dark.
The exact defition of what a Dark is has changed over the years. Originally, a Dark was the shadow version of a person. Later, it was said that everyone had a Dark version of themselves, but in season 9 Embless wondered aloud if he had a Dark version. Considering Embless' knowledge in Dark Science, this nerfs the idea.
Darks have before been defined as anyone with over 75% dark ratio.
It is unrecalled exactly when Darks first appeared, but they were definetely around by season four....