The Dark-type was added in Generation II to counteract the Psychic-type's dominance in Generation I, by being completely immune to it and able to hit Psychic-types super-effectively. It was also given to make the Fighting-type and Bug-type more dominant due to their downfall to Psychic-types in Generation I.
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.
Before Generation IV, every damage-inflicting Dark move was a special attack. All pre-existing Dark attacks that dealt damage changed to become physical in Generation IV, meaning that any Dark special attacks would be introduced from Generation IV onward. Non-damaging Dark moves work to disrupt the opponent in unique ways or raise user stats.
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.
While most types are represented quite straightforwardly (i.e. Grass-types based off plants, Fire-types being based on heat and flames) there is a rather loose definition of what represents a Dark-type. While most, such as Scrafty or Bisharp are associated with brutal, violent, "dirty" fighters (hence moves like Nasty Plot and Sucker Punch) some, such as Umbreon and Darkrai represent literal darkness, while others, such as Stunky and the Alolan form of Rattata are simply nocturnal animals. A few, most notably Absol and Greninja, are even portrayed as having noble and shady qualities. Others like Inkay and the Alolan form of Muk could reference how they produce dark things like ink and sludge. It could also describe how certain Pokémon can be sinful such as how the Alolan form of Persian is filled with pride and the Alolan form of Raticate is gluttonous.