When we look up at the Moon from Earth, we only ever see one side of it. That side has a series of flat seas of volcanic basalt that gives it the appearance of a face, the so-called “man in the Moon.” But the type of features that create that face only appear on one side of the Moon, something we noticed when we received the first images of the Moon’s far side, or “dark” side (called that not because it was actually dark, but because it was unknown at the time). That side has far less fewer seas, known as maria, and looks completely different. It was 55 years ago when we finally asked the question why, and an answer didn’t come until recently, when astronomers finally figured this mystery out.
The answer lies in how the Moon likely formed, something we also recently discovered. A large planetoid, about the size of Mars, hit Earth, causing part of it to shoot out into space, including the rocks that eventually coalesced to become the Moon. This impact heated up both the Earth and Moon, vaporizing parts of both. At the time, the Moon was a lot closer to Earth than it is now, and was gravitationally locked into its orbit, meaning that only one side ever faces Earth.
The Moon was smaller, so it cooled off faster than the Earth, but because the Earth was still hot, the part of the Moon facing our planet stayed warmer longer. This created a difference in temperature between the two sides, so their geology developed differently. The Moon’s crust, which is high in aluminum and calcium, both of which vaporize slowly, condensed on the Moon’s cooler side, but not on its warm side. This resulted in the crust of the Moon on the far side being thicker.
When large meteorites hit the Moon early on, they punched through the thinner crust on the near side of the Moon, creating the maria that we see today, the Moon’s "face." The thicker crust on the Moon's far side wasn't punctured nearly as easily, leaving the dark side cratered and craggy.
Via Penn State