I like to think of the dark side referencing the unknown or mysteries of the Force. “If one is to understand the great mystery, one must study all its aspects…”
Mysteries that the Jedi are too afraid to explore.
It is more likely the Jedi came up with the terms “light” and “dark” to reference how the two sides relate to positive and negative emotions. But Sith kept using the term in defiance of the Jedi doctrine. Or it could be a combination of both ideas.
But that’s all in-universe.