Men In Black are an evil organization according to UFO culture and conspiracy theories - although they have become noticably less popular due in part to the modern image of the Men In Black being positive (ever since the film of the same name came out, which portrayed them as heroes rather than villains).
Although seldom violent Men In Black are often said to be alien or mechanical in mannerism and often arrive at the homes of those who have publically talked about aliens or UFOs, intimidating them or asking uncomfortable questions before disappearing.
Popular Theories On Their Origins
Some believe the Men In Black are aliens in disguise who arrive in order to try and cover up their own tracks, this is claimed to be a valid explaination as to why they often show little to no human emotion as well their ability to appear and vanish almost at will from the homes of those they target.
Some believe the Men In Black are machines akin to androids or humanoid robots - which is also said to explain their lack of human emotions as well as their sometimes unusual mannerisms, however the likelihood of a human organization utilizing such advanced technologies even in the modern world is somewhat unlikely and is even more remote considering Men In Black sightings have been occuring since the 50s, when the idea of a functional android is all but impossible unless they were not of human design (or you wish to believe in alternate history).
Government Officials / Conspirators
By far the most believable (and popular) of the theories is that the Men In Black are either government officials or conspirators who are sent to the homes of individuals deemed as being a threat to the "New World Order". This would explain why many Men In Black stories involve them intimidating people rather than actually killing or assaulting them.