conspiracy theory

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A theory seeking to explain a disputed case or matter as a plot by a secret group or alliance rather than an individual or isolated act.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A hypothesis alleging that the members of a coordinated group are, and/or were, secretly working together to commit illegal or wrongful actions including attempting to hide the existence of the group and its activities. In notable cases the hypothesis contradicts the mainstream explanation for historical or current events.
• noun dismissive Hypothetical speculation that is untrue or outlandish.