Science strongly suggests the Moon was once part of Earth, formed from debris after a massive, Mars-sized planet (Theia) slammed into early Earth about 4.5 billion years ago, with the ejected material coalescing into the Moon, explaining the striking similarity in their rock compositions, especially oxygen isotopes. 

• The Collision: A protoplanet named Theia hit the early, molten Earth.
• Ejection: This immense impact blasted Earth's outer layers (crust and mantle) into space.
• Formation: This debris circled Earth and gradually clumped together under gravity to form the Moon. 

• Rock Similarities: Lunar rocks brought back by Apollo missions have almost identical oxygen isotopes and mineral compositions to Earth's rocks, as noted in NASA Science and Let's Talk Science.
• Lunar Meteorites: Rocks that have fallen to Earth from the Moon (lunar meteorites) also show these deep connections to Earth.
• Simulations: Modern computer models show this collision could have formed the Moon in mere hours, explains its orbit, and accounts for the high-energy impact needed. 

• Fission Theory: Earth spun so fast it flung off a piece that became the Moon (less favored now).
• Capture Theory: Earth captured a passing asteroid (unlikely to explain composition).
• Co-accretion: Earth and Moon formed side-by-side (doesn't fit evidence as well).