A UFO conspiracy theory is any one of many often overlapping conspiracy theories

Conspiracy theory

A conspiracy theory explains an event as being the result of an alleged plot by a covert group or organization or, more broadly, the idea that important political, social or economic events are the products of secret plots that are largely unknown to the general public.-Usage:The term "conspiracy...

 which argue that evidence

Evidence

Evidence in its broadest sense includes everything that is used to determine or demonstrate the truth of an assertion. Giving or procuring evidence is the process of using those things that are either presumed to be true, or were themselves proven via evidence, to demonstrate an assertion's truth...

 of the reality of unidentified flying object

Unidentified flying object

A term originally coined by the military, an unidentified flying object is an unusual apparent anomaly in the sky that is not readily identifiable to the observer as any known object...

s is being suppressed by various government

Government

Government refers to the legislators, administrators, and arbitrators in the administrative bureaucracy who control a state at a given time, and to the system of government by which they are organized...

s around the world. Such theories are often intentionally hoax

Hoax

A hoax is a deliberately fabricated falsehood made to masquerade as truth. It is distinguishable from errors in observation or judgment, or rumors, urban legends, pseudosciences or April Fools' Day events that are passed along in good faith by believers or as jokes.-Definition:The British...

ed, and are backed by little or no evidence, and absolutely no reliable evidence despite significant research on the subject by non-governmental scientific agencies, and therefore, are considered pseudoscience

Pseudoscience

Pseudoscience is a claim, belief, or practice which is presented as scientific, but which does not adhere to a valid scientific method, lacks supporting evidence or plausibility, cannot be reliably tested, or otherwise lacks scientific status...

.

They commonly argue that Earth governments, especially the Government of the United States, are in communication or cooperation with extraterrestrials

Extraterrestrial life in popular culture

In popular cultures, "extraterrestrials" are life forms — especially intelligent life forms— that are of extraterrestrial origin .-Historical ideas:-Pre-modern:...

 despite public claims to the contrary.