Ghosts are probably made up by humans, but that doesn't necessitate that they aren't real. I'll explain. The New Age community generally recognizes the existence of "thought forms": entities which are created in the mind, and manifest into reality. If they are real, I believe this is the best explanation for what they could be. Also, it is important to note that these "ghosts" are not human spirits or consciousness, but human egos, which is why the typical ghost is seen as an entity with an affinity towards specific places, objects, clothing, and other things of that nature. If ghost hunters are to be believed and ghosts can be picked up by instruments which measure electromagnetic fields, then this indicates that these ghosts are of a worldly and energetic nature- not an otherworldly or purely conscious nature. Indeed, a real ghost could be seen as a nuanced type of organism which is not carbon-based, and the priests who exorcise them seen as a nuanced type of big-game hunter.