Fairy does ×2 to Dark

Generations VI to IX, single types. The other way round, Dark against Fairy, is ×½.

64 Fairy and 69 Dark species among the 1025 this site covers.

Attacker, Psychic and Fairy Hatterene sprite

damage×2Generations VI to IX

Defender, Dark and Grass Wo-Chien sprite

Fairy and Dark did not exist in Generation I and Generations II to V, so Fairy against Dark has no value there.

Strongest Fairy attackers

Hatterene leads Fairy at Sp. Atk 136

Sturdiest Dark defenders

Dual types that change the answer