Men in Black (MIB), in popular culture and in UFO conspiracy theories, are men dressed in black suits who are government agents who harass or threaten UFO witnesses to keep them quiet about what they have seen. It is sometimes implied that they may be aliens themselves. The term is also frequently used to describe mysterious men working for unknown organizations, as well as to various branches of government allegedly designed to protect secrets or perform other strange activities. The term is a generic one, used to refer to any unusual, threatening or strangely behaved individual whose appearance on the scene can be linked in some fashion with a UFO sighting.
source : Wikipedia
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