Earth's moon is unusually large compared to our planet. Most moons in the solar system are much smaller relative to their planets. The leading theory is that our moon formed from a giant impact - another planet crashed into Earth, and debris coalesced into our moon.
Moons Are Everywhere
Jupiter has over 90 moons! Saturn has more than 80. Some are tiny captured asteroids, but others are fascinating worlds in their own right. Titan has lakes of liquid methane!