When something's dark, there is no light. Far from the city lights, it gets so dark at night that you can see many stars in the sky.
Dark comes from the Proto-Germanic word derkaz, which means "to hide or conceal." This meaning is preserved in the phrase kept in the dark, meaning purposely keeping information from someone. As an adjective, in addition to being the opposite of light, dark describes things that are gloomy, difficult to interpret or heavy, like a dark movie whose main character feels alienated from society.