Fascinating; I did not know this:

Billions of years ago, the moon rotated around its axis faster than it does today. But early in the moon’s history, the drag of Earth’s gravity caused the moon’s rotation to slow to what appears to be a halt from an Earth perspective. One hemisphere of the moon always faces the Earth, and the other always faces away, with Earth’s gravity acting as a tether. In other words, the moon revolves once around the Earth in the same time it takes to rotate on its own axis—a phenomenon known as tidal locking. Because of this, in 1959, although humankind had been staring up at the moon for our entire existence, we had only ever seen about half of the moon’s surface. Owing to various view angles from different parts of the Earth, the most ambitious jet-setting astronomer could have seen a maximum of 59 percent of the surface of our planetary companion. The other 41 percent was a complete mystery, hence the phrase “the dark side of the moon.”

Source: Faxes From the Far Side • Damn Interesting