The ??? type (also known as the Unknown type) is an unofficial, retired type from the Pokémon franchise. It marked its debut in Generation 2, but is retired as of Generation 5.
The ??? type solely existed in Generations 2, 3 and 4 as a placeholder type for Pokémon Eggs, and the type for the move Curse. The ??? type doesn't have any resistances or weaknesses, and no Pokémon were resistant or weak against ???. As such, there existed a ???-typed plate for Arceus to go along with its Multitype Ability. In Generation 5, Eggs no longer needed a placeholder type, and Curse became a Ghost-type move, thus was no longer needed.
In Fan games, the ??? type is sometime still used.