Answer: Think of it in this way: People often warn others not to speak badly of the Dead. Why? This is because, it is believed that their souls get "restless", and this causes them to haunt others. Thus, Dark type moves, that symbolise the "evil" aspect of the Pokemon, are super-effective against Ghosts. Type chart, effectiveness and weakness explained in Pokémon Go
Type Strong Against Weak Against
Bug Grass, Psychic, Dark Fighting, Flying, Poison, Ghost, Steel, Fire, Fairy
Ghost Ghost, Psychic Normal, Dark
Steel Rock, Ice, Fairy Steel, Fire, Water, Electric
Fire Bug, Steel, Grass, Ice Rock, Fire, Water, Dragon
Explanation: hopes this helps
