Short answers, pulled from the story.
The giant-impact hypothesis proposes that a Mars-sized body called Theia collided with proto-Earth roughly 4.5 billion years ago, and debris from that impact eventually coalesced to form the Moon. It is currently the favored hypothesis for lunar formation among astronomers.
Canadian geologist Reginald Aldworth Daly of Harvard University proposed the idea in 1946. It attracted little attention until William K. Hartmann and Donald R. Davis published supporting models in Icarus in 1975, and the hypothesis gained broad acceptance at a 1984 conference in Kona, Hawaii.
Theia is the name given to the hypothesized Mars-sized protoplanet that struck proto-Earth. The name was proposed by English geochemist Alex N. Halliday in 2000, drawing from the Greek Titan who was the mother of Selene, goddess of the Moon. Theia is thought to have originally orbited at the L4 or L5 Lagrange point of the Earth-Sun system.
Rocks returned by the Apollo program show oxygen isotope ratios nearly identical to those of Earth rocks, and a 2001 team at the Carnegie Institution of Washington confirmed their isotopic signature matched Earth's and differed from almost all other Solar System bodies. Moon rocks also contain more heavy zinc isotopes and less total zinc than Earth or Mars rocks, consistent with volatile zinc being vaporized and lost during a high-energy impact.
Giant-impact models predict that Theia's iron core sank into Earth and merged with Earth's own core during the collision, leaving the material that formed the Moon depleted in iron. Evidence from the Moon's density, moment of inertia, and magnetic induction response all indicate its core radius is less than about 25% of the Moon's radius, far smaller than the roughly 50% seen in most other terrestrial bodies.
Several compositional inconsistencies remain unresolved. The Moon's volatile element ratios are not fully explained, and the presence of water in lunar basalts is hard to reconcile with the extreme heat of a giant impact. The Moon's oxygen and titanium isotope ratios are so close to Earth's that models struggle to account for them if Theia originated elsewhere in the Solar System.