The dark side
The evil and malevolent aspect of human personality or society, often referred to in a lighthearted or comic context.
The wide adoption of the term 'The Dark Side' certainly came about as a result of the plot line of the 1977 film Star Wars Episode IV: A New Hope, written and directed by George Lucas. In that context, Lucas portrayed the 'dark side' concept as the evil aspect of the the underlying controlling power of the Universe - which he called 'The Force'. The Star Wars series has often been compared with the classic good versus evil metaphors found in earlier Hollywood films. In a parallel with early cowboy films, the film's principal antagonist, a.k.a. the bad guy, was the Dark Jedi Darth Vader. He certainly outdid any cowboy villain in the black hat department.
Lucas had other cultural influences to draw on when creating the 'dark side' concept. In the Christian imagery of the mediaeval mind, the 'light' and dark' sides of God' were Heaven and the angels and evil, sin and the Devil respectively. The radical theologian Jacob Bauthumley (a.k.a. Bottomley) wrote about this as early as the 17th century in The light and dark sides of God, 1650:
Some live in the light side of God, and some in the dark side.
The enormously successful Pink Floyd album Dark Side of the Moon was released in 1973, just when the Lucas's film was in preparation. That may have influenced the name but what is more likely to have influenced the concept was Jungian psychology's model of the shadow aspect of personality, that is, the unconscious or 'dark' side of personality. Carl Jung's work Psychological Types was translated into English by H G Bayness and published in 1923:
"For the sake of understanding, it is, I think, a good thing to detach the man from his shadow, the unconscious... One sees much in another man which does not belong to his conscious psychology, but which gleams out from his unconscious."
In 1973, the US journal The Journal of Genetic Psychology published this:
"The shadow is described as the dark side of the personality or representing the original conception of evil in the world. The latter conception places the shadow in the collective unconscious."
The current use of 'the dark side' in everyday language isn't taken as seriously as Jung, or even Lucas, might have intended it. It has the same linguistic feel as a phrase like 'be afraid, be very afraid', which is never used in circumstances that are actually frightening . Someone might humorously be said to be 'going over to the dark side' if they opt for a preference for something that is out of fashion or not a social group's accepted preference - like a Londoner supporting Manchester United, for example.