The Dark type is easily the most OP type there is! It has tons of good attackers, supporters, mixed and so on, as well as tons of good moves WITH really good secondary effects, lots of support moves and so on! It's too strong to put in a Gym early on, but for later on, there are other types, with more bulk, that would do a much better job at battling, like Dragon, Fairy, Steel and Ice, leaving little room for it, as a Dark type expert will need to have both offensive and support moves, which is impossible due to the low AI of the Gym Leaders. However, when it comes to the Elite 4, the Dark type is perfect! They have a much better AI than the Gym Leaders, allowing for better strategies and battling! Additionally, facing something this strong anywhere but in the Pokemon League, wouldn't be as satisfying to beat! What I would really like though is, not a Dark Gym, but a Champion that is an expert at Dark types and also with a really strong team!