I'm not going to flat out say they don't exist, because with all the advances in science we are making, who knows what we'll discover? As for now, I don't believe they exist. Peoples' imaginations really do run wild in the dark and with things they don't understand. This is one of the reasons humans invented religion. Yes, sometimes it feels like I'm being watched or that "something" could be there, but it's just my mind running amok trying to deal with an uncomfortable situation.