Which is the primary theory for how the moon was formed?

Meteor that hit the earth.

In the 1970s, two sets of astrophysicists independently came to the conclusion that the Moon was created by a glancing collision between a Mars-sized object and the still-forming Earth. The massive impact theory explained many things, like the large size of the Moon in relation to the Earth and the rotation rates of the Earth and Moon, and it gradually became the primary theory for the Moon's formation.