Dark does ×2 to Psychic

Generations VI to IX, single types. The other way round, Psychic against Dark, is ×0.

69 Dark and 102 Psychic species among the 1025 this site covers.

Attacker, Dragon and Dark Roaring Moon sprite

damage×2Generations VI to IX

Defender, Psychic and Flying Lugia sprite

Lugia #0249

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

Strongest Dark attackers

Roaring Moon leads Dark at Atk 139

Sturdiest Psychic defenders

Dual types that change the answer