It was recently revealed that Pokémon Sword & Shield will include eighteen Pokémon Gyms, which is also the number of different Pokémon types that currently exist in the series. It seems likely that Pokémon Sword & Shield will be the first game in the series to include a Dark-type Gym.

The Gyms in the Pokémon series have represented every type of Pokémon except for Dark-type. Pokémon X & Y had a Fairy-type Gym, which made sure that the type was represented in the game where it was introduced, while Pokémon Ruby & Sapphire had a Psychic-type Gym in Mossdeep City that also focused on Double Battles.

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There have been several Elite Four members who have specialized in Dark-type Pokémon (Karen from Pokémon Gold & Silver, Sidney from Pokémon Ruby & Sapphire, and Grimsley from Pokémon Black & White), but there hasn't been a single Dark-type Gym Leader to date.

via pokemon.wikia.com

It's believed that a Dark-type Gym was originally planned for Pokémon Gold & Silver, but was scrapped during development. The Gyms in the Johto region in Pokémon Gold & Silver all belonged to types that weren't represented in Pokémon Red & Blueas they also appeared during the second half of the game. The leaked demo of Pokémon Gold & Silver from Space World '97 featured a different line-up of Gym Leaders and the original fifth Gym Leader was named Okera. The name Okera is similar to the name of a type of demon from Japanese mythology, so it has been speculated that he was the leader of a Dark-type Gym that was scrapped.

It's unclear why Game Freak has waited so long to include a Dark-type Gym in the Pokémon series, especially as there has been an abundance of Fighting-type and Water-type Gym. The fact that Pokémon Sword & Shield is including the same number of Pokémon Gyms as there are types suggests that it will be the first game in the series to include the long-awaited Dark-type Gym, though it remains to be seen if it will be part of the Major league or Minor league of the Galar region.

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