Evidence behind Giant Impact hypthesis The giant impact hypothesis is a theory to explain how the moon was formed. What evidence is there that supports the theory and led us to find the current theory?
The smoking guns:


The ratio of oxygen isotopes of lunar rocks are almost identical to those of Earth.
Lunar recession due to the tides which causes the Earth's rotation to slow down, means that just after the Earth formed, the Moon was very close to the Earth and the Earth was rotating very fast. This situation can be reached due to an oblique impact that transfers a lot of angular momentum to the Earth.
The formation of the planets is known to have proceeded via small objects growing larger by colliding with each other, this means that the last stages of planet formation would have involved collisions between large objects. It is to be expected that around the time the Earth was taking shape, there could also be a proto-planet at the Lagrange point 60 degrees away from Earth in the same orbit. On the long term this is an unstable situation which ends with a collision that according to simulations is of the sort that typically leads to the formation of the Moon.
The asymmetry in the geology between the far side and the near side of the Moon. (there are many more maria on the near side compared to the far side). If the Moon had indeed been very close to the Earth after it formed, then the near side would have stayed very hot due to the Earth's surface still being molten and the radiant heat affecting the near side of the Moon. An impact on the near side of the Moon would far more easily be able to penetrate the solid crust and cause magma to flow to the surface compared to the far side of the Moon.