Out of the debris of this massive impact, the Moon was formed.

This theory fit most of the facts, but not all of them. In particular, it didn't fit with the composition of isotopes found on the Earth and Moon. Isotopes are a very useful for finding where things come from because their ratios are pretty much set at an object's place of origin like a nuclear bar code. By proper measurement, scientists can tell everything from whether a meteor came from Mars to which farm a chicken was raised on. Unfortunately, the Moon turned out to be a bit of a poser.

If the Moon was created by Theia striking the Earth, then the the isotope ratios of the Earth and Moon should be very different, but they're not. They're similar – too similar to have a separate origin.