The Dark type was added in Generation II to cause a weakness on Psychic types, which had the weaknesses to Bug type (the moves were really weak to cause a major damage) and Ghost type (the only move was Night Shade, and there was a glitch that prevented Ghost moves from hitting Psychic Types).
The Dark type is represented from traits that are considered feral and untameable, specifically that born from adverse and intense complex social conditions, to that of traits that evoke a negative leaning nature, from having a nocturnal day cycle, exhibitions of unusually cruel, crafty, and clever intelligence, belligerent aggressiveness, the sentient awareness and choice to harm and/or inflict ill will, and to that of the negative end of nature and reality, such as the shadows, declination, "the survival of the fittest", destruction, comings of darker times, and to the unknown but potentially dangerous aspects of the mysterious and enigmatic. It should be further noted that the type of Dark does not automatically associate with the concept of evil, as both evidence exists of cruelty capable of Pokemon not just of the type (Ekans, Fearow) and that it is the trainer that influences majorly how a Pokemon matures and acts.
Many Dark types are rather just born with features that define them as a type, as they understand of the powers that come with it, such as Absol and Sableye, and/or are defined so by the perspective of humans with necessary purpose in the environment and their species but whose actions and traits are at most taboo and are discouraged in human society.
It's hard for a Dark-type Pokémon to form bonds of friendship with its Trainer, but when having one, it will never break, being really faithful to its Trainer and protecting it anytime.
The Dark-type Pokémon, along with the Poison types, are mostly used by the criminal organizations.
They're strong against the Psychic type because of the fears, and the fears can affect one's rationality and concentration. The darkness of one's being/psyche is also explored to utterly be taxing to those who can read minds and hearts, as it is regarded to be a place to never cross into and can violate one's own, to where ones of unusual nature are not easily understood, and of how there are legends and themes that show that times of evil and darkness cannot be divined nor preempted by even the best seers and prophets.
They're strong against the Ghost type because evil can damage the soul.
They're weak against the Fighting type because philosophically, a warrior must overcome their fears - represented by Dark. The Fighting type also supposedly represents justice, which opposes evil, also represented by Dark types.
They're weak against the Bug type because bugs operate well in darkness.
They're weak against the Fairy type because fairies represent light and the darkness disappears with light. Furthermore, light is a metaphor for goodness, which always triumphs over evil in fiction.
If a Dual-type Pokémon is both Dark and Ghost type, both types cancel out each other's weaknesses with their resistances, leaving Pokémon like Spiritomb and Sableye with no weakness apart from Fairy.
So far this is the only Pokémon type not to be used in a Pokémon Gym.
The Dark type has the same efficacies over other types as the Ghost type.