The leading theory, now backed by decades of evidence, is the Giant Impact Hypothesis. It goes like this:
In the early, chaotic days of the solar system, a Mars-sized protoplanet - named Theia - was on a collision course with young Earth. It didn't just graze us. It slammed into our world with unimaginable force, vaporizing itself and blasting a massive cloud of debris into orbit.
That debris, over millions of years, coalesced. It cooled. It became the Moon.
Think about the scale of that violence. An entire world - half the size of Earth - obliterated in a single, apocalyptic moment. The impact was so powerful it tilted our planet's axis, giving us seasons, and set the stage for everything that followed.
But here's the haunting part: Theia didn't vanish.
Most of it was absorbed into Earth. Mixed into our mantle. Melted into our core. Theia is still here, hidden beneath continents and oceans, fused with the very ground you walk on.
And the Moon? It's the surviving witness. The debris that didn't fall back to Earth. The cosmic scar tissue from the most significant event in our planet's history, hanging silently in the night sky.
The strongest evidence? Oxygen isotopes. The Moon rocks brought back by Apollo astronauts share the same isotopic fingerprint as Earth. They came from the same source. They are, in a very real sense, pieces of the same cataclysm.
So the next time you look at the Moon, remember: you're not just seeing a distant rock. You're seeing the ghost of Theia, the world that died to create it. And every step you take on Earth is a step on the remnants of that long-lost planet.
The Moon isn't just our neighbor. It's our sibling, born from the same violence that made us.