Either may be correct, in different circumstances.
In speaking of facts about a past event or a dead person, use the past tense: there is strong semantic dissonance in saying
∗ I found out that Shakespeare is a male.
∗ She said that the Battle of Borodino is fought on September 7.
But when you are speaking of enduring truths—facts which would in immediate discourse be reported with the present tense—you have a choice. Which tense you use will depend on the context: what are you going to do with what was found out or said or shown?
If you are dealing with the present consequences of the discovery/claim/demonstration, you should you use the present tense:
She said that J.K.Rowling is male, which we all know to be false.
Galileo showed that the earth revolves around the sun, which we all take for granted today. †
But if what was discovered/claimed/shown is no longer true, or has been shown to be untrue, or if you are narrating what resulted in the past from the discovery/claim/demonstration, you have the option of using the past tense, which may provide your discourse a little more coherence:
It used to be said that the subject of a gerund clause had to be in the possessive case.
When as a boy I found out that the author of Harry Potter and the Sorcerer’s Stone was female I started taking women writers more seriously.
Galileo showed that the earth revolved around the sun, which excited considerable hostility among theologians.
∗ marks a usage as unacceptable
†Thanks to BobRodes for this example.