Well, this is actually a bit simple: You see, the earth revolves around the sun, and the moon revolves around us. As we spin, the moon spins too. This is due to gravity. The issue is the moon has the EXACT orbital period as us, meaning we will always be facing the same side of the moon, because it's spins are perfectly timed with ours. If the moon didn't spin at all, then we would EVENTUALLY see the "dark side of the moon" but because it does spin, we are always stuck seeing the same side.
