Answer: Actual scientific theory!
Although he's a genius, Sheldon Cooper's brash attitude can sometimes have an enormous aftermath on those around him, so it would have made total sense for an episode to be named "The Giant Impact Hypothesis" and be all about his action's unintentional affects on his many relationships.
Alas, "The Giant Impact Hypothesis" is so much bigger than Sheldon and his larger-than-life personality. This actual hypothesis suggests that our moon was the product of an indirect collision between Earth and a massive celestial body that took place about 4.5 million years ago.
Giant impact, indeed!