What are some of the big-time conspiracy theories that turned out to be true & accepted?

There are so many conspiracy theories regarding people such as JFK, Kurt Cobain, Elvis, or events like 9/11.

Was there ever a conspiracy theory about as big as any of these that turned out to be true (AND accepted by the majority of people)? If so, please tell.

Thanks.

The Watergate Scandal was a hidden conspiracy that when unraveled toppled a presidency.