Title says it all, Im baked off my fucking head. Gotta say, I've actually scared the shit out of myself asking this question.
Not in the religious "cloved feet, horned nasty motherfuckers" , but I definitely think there's malevolent 'energy' in the universe that can influence a person if exposed. But the real demons are man made. Drugs such as heroin and crack which change the personality of those who take them. The pressures of debt. The guilt of murder. But I definitely think there's a race of aliens out there on some death world who would match the biblical definitions of a demon.
Go check out /r/ No Sleep - Because you weren't planning on sleeping anyway Then demons will be the least of your worries.
People are demons. A-level demons = rapist, murderers, extortionist, dirty cops B-level demons = major thieves, physical bullies, heart breakers C-level = minor thieves, mental bullies The worst your karma the more power these "people" will have over you. Just a theory.