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. Since Generation VI, it is weak to Fairy 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 Pokémon not just of the type (Ekans, Fearow) and that it is the trainer that influences majorly how a Pokémon 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-type moves were special moves. 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 people's fear of either darkness or evil people, and fear is known to disrupt one's rationality and concentration. It can also be because of how easy it is to corrupt one's mind with wicked and malevolent thoughts, which is why Dark is the only weakness of Psychic to be immune to it as well.
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 some bugs operate well in darkness. While criminals are active during darkness, when they least expect it, bugs can swarm over them.
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.
Before Generation VI, the Steel type resisted them; this is likely in reference to the idea of steel being used in security measures, like locks or even weapons for self defense, eliminating threats and practicing skills, and to how some metals like silver are weaknesses of classical monsters and demons. In turn, so may the crafty and clever find ways to overcome security measures with figuring out their features.
They resist themselves because it's hard for villains to recruit anyone who is already evil or when they have expressed disapproval towards them for their certain personalities or actions.
Trivia[]
If a Dual-type Pokémon is both Dark and Ghost type or vice versa, both types cancel out each other's weaknesses with their resistances, leaving Pokémon like Spiritomb and Sableye immune to super effective damage until the introduction of the Fairy type in Generation VI.
Prior to Generation VIII, Dark type was the only Pokémon type not to be used in a Pokémon Gym. Because of this, the Dark type has the fewest Gym Leaders.
The Dark type has the same type advantages as the Ghost-type (Psychic and Ghost).
While most types are represented quite straightforwardly (i.e. Grass types based off plants, Fire types being based on heat and flames, etc.) there is a rather loose definition of what represents a Dark type. While most, such as Scrafty or Bisharp are associated with nasty, "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, the Alolan form of Raticate is gluttonous, and the Galarian form of Moltres is wrathful.
The original Japanese name for the Dark type is actually the "evil" type.
The Vullaby line are the only Dark type Pokémon to be exclusively female.
The Impidimp line are the only Dark type Pokémon to be exclusively male.
The Dark-type is the only type that cannot be obtained in any Kanto-based game (those being the Gen I games, the Gen III remakes, and the Gen VII remakes) without trades or, in only one specific case, Mega Evolution.