This is where the Earth was hit by a large planet (Theia), usually said to be about the size of Mars, and the resultant debris and the Earths gravity formed the Moon.
There are a lot of issues and evidence against this large theia impact happening but planetary formation theory does not seem to have many alternatives that fit into a Gravity Universe, so they are considered as problems to be solved.
Moon formation theories problem?
A new theory about how Earth lost its original atmosphere and how it was replaced has recently been announced by MIT and Caltech researchers. In this report they mentioned about their results for an impact similar to the planet Theia on the Earth. But it seems to suggest that no massive direct impact could or should have occurred.
A collision with an impactor as massive as Mars, the researchers found, would generate a shockwave through the Earth’s interior, setting off significant ground motion — similar to simultaneous giant earthquakes around the planet — whose force would ripple out into the atmosphere, a process that could potentially eject a significant fraction, if not all, of the planet’s atmosphere.
However, if such a giant collision occurred, it should also melt everything within the planet, turning its interior into a homogenous slurry. Given the diversity of noble gases like helium-3 deep inside the Earth today, the researchers concluded that it is unlikely that such a giant, core-melting impact occurred.
Unless the difference is going to be one version of the Theia planet was a glancing blow, but still enough to have ejected all the material that would have escaped the Earths gravity and enough left over to form the Moon, and the MIT/Caltech study is a more straight on blow, that would have still ejected material etc?
Moon formation theory in an Electric Universe?
The EU theory suggests that the Moon was never formed out of material from the Earth, that the Earth captured the planet that become our Moon.