Currently, the best explanation for the origin of the Moon is the theory of the collision of two protoplanetary bodies in the early solar system accretion. The "Great Collision Theory", which became popular in 1984 (although it was created in the 1970s), fulfills the conditions of the Earth's or Moon's orbit and explains the relatively small metallic nucleus of the Moon.
The collisions between planetesimals are now considered to be the cause of planetary bodies growing in the early stages of solar system evolution, and with this assumption larger collisions are inevitable when the planets are almost completely formed.
The theory assumes a collision between the body with a diameter of 90% of the present Earth and the size of Mars (half of the Earth's radius and tenth of Earth's mass). The name of the alleged body that hit the emerging Earth - Thea - was given to the myt.