How was our Moon formed?

The answer

The leading theory of lunar formation is the Giant Impact Hypothesis. About 4.5 billion years ago, a Mars-sized body called Theia collided with the early Earth at an oblique angle. The colossal impact liquefied both bodies and ejected a vast cloud of debris into orbit around Earth. This debris gradually coalesced to form the Moon. This theory explains why Moon rocks brought back by Apollo astronauts have a chemical composition remarkably similar to Earth's mantle, and why the Moon has a comparatively tiny iron core - most of the iron stayed in Earth's core during the impact.