From Bulbapedia, the community-driven Pokémon encyclopedia.
Dark City (Japanese: ダークシティ Dark City) is an anime-exclusive location in Kanto found in between the Safari Zone and Cinnabar Island.
The city does not have an official Pokémon Gym, but two rival Gyms seek the wealth and glory that comes with having an official gym. When Ash and friends visited the city in Showdown at Dark City, the city was shown to be in disrepair because of the constant warfare from the competing Gyms. The residents of this town live in fear of Pokémon Trainers.
Neither the Yas Gym nor the Kas Gym have a specialist type; their leaders' main Pokémon are Scyther and Electabuzz respectively. For this reason, their followers do not appear to join for personal connection with a type of Pokémon (like in most gyms), and the actions of the Yas Gym Leader and the Kas Gym Leader appear to intimidate trainers into joining their Gym (lest they join the rival Gym).
After an agent for the Pokémon Inspection Agency determined that neither Gym was worthy of being an official Gym, the two rivals agreed to stop fighting and work together to rebuild the town that they had destroyed.