According to the reigning hypothesis, about 4.5 billion years ago, shortly after Earth had accreted down into a sphere from its little slub of circumsolar material, another newborn planet, still shaky on its feet, slammed obliquely into Earth with terrifying force. That "giant impactor", named Theia, who in Greek mythology was mother to the goddess of the moon, is thought to have been roughly the size of Mars and to have been pulverized in the encounter, along with a good chunk of the proto-Earth. From that fiery cloud of all-Theia and part-Earth, the scenario goes, our moon soon condensed.
2014 September 7, Natalie Angier, “The Moon comes around again ”, in The New York Times