This is the popularly accepted the theory for the formation of Moon:
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 aggregated into the moon
Check out these links to find more about the scientific theories for the origin of the moon
http://www.psi.edu/projects/moon/moon.html
http://csep10.phys.utk.edu/astr161/lect/moon/moon_formation.html
