MIB

MIB is an acronym standing for Men In Black.

Men In Black are usually government agents carrying out deliberately mysterious campaigns of propaganda, agitation or disinformation, usually against Fortean researchers or UFOlogy cultists. Although this is the most common cause of MIB reports, it is a documented fact that sometimes there are far stranger explanations for the appearance of MIBs.

http://www.aliens-everything-you-want-to-know.com/TheMeninBlack.html

https://listverse.com/2014/05/27/10-creepy-stories-of-encounters-with-men-in-black/

The first widely
acknowledged case of Men In Black
intimidating UFO witnesses occurred in 1953. A fellow by the name of
Albert K.
Bender was the editor of a magazine called Space Review. He was also
the
founder of an organization called the International Flying Saucer
Bureau.

During the summer of
1953 Bender apparently discovered
some vital information pointing to the cover-up of the existence of
flying
saucers by the U.S. government. He had written several articles
scheduled to
appear in the next issue of his magazine. The next thing he knew, three
guys
show up at his door all dressed in black: black suits, black hats, and
sunglasses. They told him they had read his article, even though it had
not yet
been published. They told him his information was accurate, but that he
better
not publish the article. In fact, they told him that he'd better not
publish
anything more about flying saucers. They said, "We advise those engaged
in
saucer work to please be very cautious." They basically scared him so
badly that Bender officially retired from UFO investigations.

Among the early victims of this evil "Silence Group" was Albert K.
Bender of Bridgeport, Connecticut. In 1952 Bender formed the
International Flying Saucer Bureau (IFSB), which met with immediate
success, but he shut it down the next year under mysterious
circumstances. In due course Bender confided that three men in black had
 imparted to him the terrifying answer to the UFO mystery and turned his
 life into a nightmare. He would say no more. Three years later an IFSB
associate, Gray Barker, wrote a book about the episode; the title
perfectly captured the paranoia abroad in UFO-land: *They Knew Too Much About Flying Saucers*.

Through the "Bender mystery" the legend of the "men in black"
(MIB) came into the world-even though, as Barker observed, a man in
black had played a villainous role in the Maury Island incident.
 According to Barker, the MIB were ranging as far afield as Australia
and New Zealand, scaring still more UFO buffs into silence.

 By the late 1980s MIB tales had become sufficiently ubiquitous
that the august Journal of American Folklore took note of them in a long
 article. Just who the MIB were remained unclear. To saucerians enamored
 with conspiracy theories,
 they were enforcers for the Silence Group, associated with
international banking interests that sought to stifle the technological
advances and moral reforms the Space Brothers wanted to bestow on
Earthlings. To others, they were alien beings-perhaps, some speculated,
Shaver's deros. In 1962 Bender came down on the side of the alien
school. Breaking his nine-year silence in *Flying Saucers and the Three Men*,
 which he insisted was not a science-fiction novel, Bender revealed that
 the men in black who drove him out of ufology were monsters from the
planet Kazik. Even Barker, the book's publisher and a relentless Bender
promoter, remarked privately and out of customers' hearing, that maybe
it had all been a "dream."