The theory is that much of Theia was absorbed in to the Earth and the mass that was ejected during the collision became the moon. https://en.wikipedia.org/wiki

The theory is that much of Theia was absorbed in to the Earth and the mass that was ejected during the collision became the moon. https://en.wikipedia.org/wiki/Theia_(planet) https://en.wikipedia.org/wiki/Theia_(planet)