Has any conspiracy theory turned out to be true?

There are various lists of conspiracy theories that turned out to be true:

The implication would seem to be lend credence to current conspiracy theories. But these lists tend to contain many entries which were either not about conspiracies, were never part of a conspiracy theory, or are still quite dubious.

Are there any examples of a theory which:

  1. Postulates a large-scale conspiracy, that is, a significant group of people having a shared secret plan to cause harm or commit a crime.
  2. Was believed by a vocal minority, but widely regarded as nonsense.
  3. Eventually turned out to be correct, such that virtually everyone agrees with it now.

closed as primarily opinion-based by Christian, jwenting, Sklivvz Aug 12 '16 at 21:48

Many good questions generate some degree of opinion based on expert experience, but answers to this question will tend to be almost entirely based on opinions, rather than facts, references, or specific expertise. If this question can be reworded to fit the rules in the help center, please edit the question.

  • 2
    This is a question on definitions, closing. – Sklivvz Aug 12 '16 at 21:49
  • Comments are not for extended discussion; this conversation has been moved to chat. – Sklivvz Aug 13 '16 at 11:03
  • It seems to me that conspiracies by definition are no longer conspiracies once pain facts are easily found. But as has been said much in the comments that have been moved to chat, there are many examples of things originally denied by let's call it mainstream, especially regarding medicine and health. Does this actually count as a conspiracy being proved? That's really the issue with this question. – fredsbend Aug 13 '16 at 15:55
  • @fredsbend, I was aware of that problem and tried to forestall it by defining what I meant by conspiracy theory with the criteria in my question. That the way the question would be objectively about whether something fit my criteria, not the subjective notion of whether or not a conspiracy theory has been proven. But I wasn't clear or precise enough in my question, so that's that. – Winston Ewert Aug 13 '16 at 20:39

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