Dark does ×2 to Ghost

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

69 Dark and 65 Ghost species among the 1025 this site covers.

Attacker, Dragon and Dark Roaring Moon sprite

damage×2Generations VI to IX

Defender, Poison and Ghost Pecharunt sprite

Dark did not exist in Generation I, so Dark against Ghost has no value there.

Strongest Dark attackers

Roaring Moon leads Dark at Atk 139

Sturdiest Ghost defenders

Dual types that change the answer