The term "Men in Black", or MIBs in UFOlogy circles, refers to alleged incidents of men dressed entirely in black claiming to be government agents, who attempt to harass or threaten UFO witnesses into silence. This phenomena was initially and most frequently reported in the 1960s; it is contemporaneous with many other conspiracy theories. The probable origin of MIBs are the (existing) police forces that protect the NSA headquarters in Maryland, who wear black paramilitary uniforms.
As the story goes, shortly after a (presumably credible) witness reports a UFO sighting, speaks to the media, or produces photographs, the witness is paid a visit. If not alone, two or more males, always in even numbers, always dressed almost entirely in black, arrive in a large black vehicle, typically a Cadillac. Their style of dress and make of vehicle is usually described as slightly out-of-date, perhaps by 10 years, and they are often noted as having odd mannerisms. Usually introducing themselves as agents of an undisclosed government agency, they flash convincing looking badges and demand that the witness recant their story, or hand over anything like photographs, or physical evidence of a UFO. If the witness refuses or questions their credentials, they often subtly or not-so-subtly threaten the witness or their family with bodily harm.
While it is not known if these threats have ever been realized, resisters have reported subsequent encounters where they have been chased or roughed up by the "agents". Houses have allegedly been ransacked or burned down, in an attempt to destroy evidence or scare the witness into silence.
The actuality of the phenomenon is highly questionable. The depth of the conspiracy theory leads some to believe that their odd mannerisms and dress are due to the fact that they are aliens or alien-human hybrids, and that their job is to eliminate physical evidence of alien involvement on earth. Others believe that they are actual government agents who intentionally dress and act ridiculously, in an attempt to get UFO witnesses to discredit themselves if they ever report such an encounter. Most people, however, give very little credence to the belief. It does seems to have some slight basis in fact however, as on more than one occasion police have chased what some believe to have been MIB vehicles.
See also Anomalous phenomena
Also an album by The Stranglers.