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.
I think this is the best explanation of the use of ‘Dark Side’, but this would require the context of Sith dropping the term in favor of something else when not in the presence of Jedi. After all, according to Palpatine the Sith use a wider array of emotions (and presumably positive emotions) than can be encapsulated by the term. In this way the Sith’s philosophy would become genuinely seductive because of its all-inclusiveness.