The Cosmic type is one of the many fanon types from the Pokémon franchise. They are generally an defensive type, with a sort of focus on evasion, lowering of accuracy and the like. It was added at the same time as the Sound type and can be considered the counter to it, hiting it with supereffective damage and being immune to it's attacks. Notably, Cosmog and its evolutionary line replaces the psychic type with this type, and Cosmog would also have the Gas type, making Cosmog a Cosmic Gas type.
The common Cosmic-type Pokémon, however, has a few weaknesses. While being able to resist and weaken the Sound type, it is generally weak to itself, Electric and Psychic, with the last two being strong Special Attackers; however, this is combated by a strong Special Defense stat, generally canceling out eachother.
A Cosmic-type Pokemon typically has strong Sp. Defense and Defence, mediocre HP and Sp. Attack, and low Speed and Attack.