Let's take one of the older conspiracy theories: 9/11 was an inside job.

* "something that is known to have happened" - Yep, 9/11 did happen. And some people say they KNOW that it was an inside job.

* "especially something for which proof exists" - job of the CIA is to exactly prevent such a thing, they did not.

* "or about which there is information" - come on. I won't link conspiracy theory websites here.

So, in the end, your definition does not contradict that someone calls the 9/11 conspiracy theory a "fact".

The first point is debatable - but then you could apply the same logic to WW1 or any other history event.

Therefore, I don't like your definition of fact.