The currently accepted theory of the moon's origin is;
The giant impact hypothesis.
At the time Earth formed 4.5 billion years ago, other smaller planetary bodies were also growing. One of these hit earth late in Earth's growth process, blowing out rocky debris. A fraction of that debris went into orbit around the Earth and eventually aggregated into the moon.
The Earth has a mean density of 5.5 grams/cubic centimeter, while the moon has a density of only 3.3 g/cc.
The Earth's high density is due to its iron core.
The Moon's low density implies that the Moon does not have an iron core.
The giant impact hypothesis explains the Moon's lack of an iron core by assuming that the iron core's of both the proto-Earth and the impactor coalesced at the time of impact, while a large amount of the rocky mantle was blasted into space. This rocky debris later aggregated into the moon.
The moon has exactly the same oxygen isotope composition as the Earth, whereas Mars rocks and meteorites from other parts of the solar system have different oxygen isotope compositions.
The standard interpretation of the oxygen isotope data assumes that the material forming the proto-Earth and the impactor were derived from the same source region.
However, impact simulations which are successful in forming the Moon suggest that most (about 80%) of the proto-lunar material is derived from the impactor, not the Earth. In this case, even small isotopic differences between the two colliding bodies should have left observable signatures between the two resulting bodies, because the impactor-derived lunar material escapes the diluting effects of mixing with the largest reservoir in the system, the Earth.
Thus, it is not good enough for the impactor to have formed in an orbit around the Sun, but close to Earth's, the impactor actually had to be a binary of the Earth.
So, the Moon had to be formed from the impact of a former moon,... which is not really satisfactory for a theory which is meant to explain the origin of the Moon, is it?
Actually, this is exactly what is assumed in one version of Kevin Mansfield's collision hypothesis.
Another attempt at a solution is outlined in;
but is very far from convincing.
So, what do you think?