Where did the Moon come from?

You would think we knew that, wouldn’t you? But do we? Silly answers aside, what do you think its origin is?

The Moon’s materials and makeup match Earth’s in some ways we do not see in, say, material found floating in the asteroid belt or objects that enter the Solar System from distant space. It has lots of oxides, for instance, indicating that oxygen and other gases were present in its formative stages. It’s oxygen isotopes are identical to those of Earth. It has water, now frozen, and may well have had more that was lost into space due to its relatively small mass, and thus low gravity. But it has no iron core as Earth does, and thus a much lower density (3.346 g/cc versus Earth’s 5.519 g/cc).

How old is the Moon? Why does it share some features with earth, but differ so substantially in other features? How did it get where it is today?