The Moon is thought to have formed nearly 4.5 billion years ago, not long after Earth.
The Moon after Jupiter's satellite Io, is the second most dense satellite among those whose densities are known.
The side of the Moon that faces Earth is called the near side, and the opposite side the far side. The far side is often inaccurately called the "dark side", but in fact, it is illuminated as often as the near side: once per lunar day, during the new moon phase we observe on Earth when the near side is dark
http://en.wikipedia.org/wiki/Moon
But we still do not know how the Moon was actually formed. None of the usual theories actually work - let';s take a quick look at them:
1 - The Fission Theory. This was the first attempt at explaining the origin (George Darwin in 1878), and supposed that originally the Earth & Moon were one object, spinning rapidly & that solar tidal action caused a Proto-Moon to fission off from the proto-earth. This one survived until the 1920's, when it was falsified by a British Astronomer who pointed out that the viscosity of the earth in semi-molten condition would have dampened the motions necessary to trigger Darwin's fission.
2 - The Co-Accretion Theory, which is falsified by angular momentum issues.
3 - The "Capture" Theory, which is an absurdity because of orbital problems (more on this later)
4 - Big Whack Theory. This is the latest one. It too has problems, and suggests the early earth was heavily impacted by a Mars sized object and the biggest (of many) problems is that such an impact would have greatly speeded up Earth's rotation - this is admitted by the theory proponents, who hypothesize a second equally massive impact at the opposite side of the planet to stabilise it again.
Lunar Anomalies:
1 - It is by far the biggest moon in the solar system by quite some way - this is what falsifies the "capture" theory in part as it is too large to be in the orbit it is if captured.
2 - It is 400 times smaller than & 400 times closer to the earth than the Sun is.
3 - The above is a unique condition that creates Solar Eclipses - a very unlikely phenomena.
4 - It's actual composition is very different to that of the Earth, although oxygen isotopes suggest it formed about 4.5 Billion years ago.
5 - It's Rotational period is almost the same as it's orbital period. This is why it always presents the same face to us.
6 - Why no Mare on the far side?
7 - What creates the Mascons noticed by NASA?
8 - Why, when the Apollo 13 module was dropped back onto it, did it indeed "ring like a bell".....for all the world as if it were hollow indeed.
The biggest mystery of all is why we have never been back since 1972, given (a) the vast mineral wealth up there - The moon's crust is loaded with Titanium & Chromium and (b) NASA are seriously looking at capturing & towing an asteroid into earth orbit for mining purposes.....