Brace yourselves, a TL;DR post about the absence of Dark Gym is coming...
I really don't believe no-one has figured out already the reason for the absence of a Dark-type gym.
Let's start with a little story. Gym leaders are believed to be the guardians of peace and quiet in their respective cities. When there's no challengers, they walk around helping every one in need, and fight off evil and criminal organizations.
Kanto is one of the oldest regions of the Pokémon World, and its gyms have a peculiarity: all gyms were first created, than organized in a League. That's why Koga, a Ninja, and Sabrina, who stole the title of official Gym of the city, are leaders, by sheer force. But Koga got tired of helping people out and decided to challenge the E4 and become a member of it.
All other regions have leagues that vote for Gym leaders, and decide whether someone can or cannot become a leader. This person should always be helpful and ready to take care of his/her city.
OTOH, E4 members are only the strongest trainers in the continent, they don't need to care about anyone. With the exception of the Champion, who is not technically part of E4 and generally flies everywhere to help Gym Leaders with their jobs, all the others only care about their business while waiting for challengers, be it training hard in mountains, writing a book, practicing Ninja techniques or sleeping.
"Why the hell are you telling this, if everyone knows it?"
Simply because Dark type is called "Evil" in Japanese. And since people who are specialist trainers share a strong link with their types, no organized league will allow someone so deeply entangled with Evil Pokémon to become responsible for the welfare of a city and its people.
That's not to say Dark trainers are evil, but they're generally selfish, egocentric and focused on improving their own power, rather than taking care of burned towers or of special sanctuaries. The same goes or Dark Pokémon. Absol is white and a Dark type, because it's a warrior of Light fighting among the shadows, bringing the destruction that renews everything.
(sorry for it, I'm really into the philosophical part of the in-game story)
TL;DR: Dark is called Evil in Japanese, so there can't be an Evil leader.
