At these scales, there are no rigid bodies, and so no single "chunk". If the giant-impact account is correct, the moon coalesced out of ejecta in orbit. > And

At these scales, there are no rigid bodies, and so no single "chunk". If the giant-impact account is correct, the moon coalesced out of ejecta in orbit. > And why wouldn't the chunk either escape or fall back down, rejoining the main mass? Some material likely did escape, but that takes much more energy than getting into orbit. You need to leave the surface of the Earth at about 11km/s to escape, but only 6km/s to reach lunar orbit.