The Great Impact hypothesis is a hypothesis that is explaining how the Moon was formed. According to it, the Moon was formed after a collision between the Earth and another planet that had approximately the size of Mars, somewhere during the Hadean eon.
The Moon is a weird natural satellite when compared to the other satellites in the solar system. It is way to large compared to the planet around which it orbits, the Earth. Despite its big size, it has very low mass, as well as very low gravitational pull.
The Great Impact hypothesis manages to explain this. When Earth and the other planet (called Theia) collided, material from the surface of both planets was detached from them. This material gave the foundation basis of the Moon. The material on the surface is always the lighter material with lesser density, and since the Moon is thought to be made out of this material, it is logical that it will have very low mass for its size.
