The far side of the Moon was not seen by the public until 1959. The Moon is pointed away from us when it rotates once a year because it keeps one side pointed toward us.
For the first time, we got a glimpse of the landscape that was shrouded in mystery until the Soviet Union sent the Luna 3 spaceship. The side we can see is made up of rough, cratered highlands and smooth, dark low areas, created from lava flowing across the surface. These are called maria because they look like bodies of water.
There is only one small mare on the far side, and the rest is covered in craters. Other differences were found as technology improved. The near side has more of the elements thorium and titanium than the far side, which is called KREEP terrane. It was discovered that the far side is thicker than the near side.
Some pretty clever ideas have been proposed to explain this. After a Mars-sized planet whacked the Earth hard enough to cause enough material to coalesce and form the Moon, it formed two moons; a big primary one and a smaller one. The thicker crust on the far side was formed by the second impact. When the Moon formed, it was so close to Earth that the still-hot-from-the-giant-impact planet heated it, causing material to flow around to the Moon's far side. This is linked to more volcanism on the near side, which changed the elemental abundances.
A new idea has just been published by planetary scientists, and it implicates a huge impact on the Moon that changed the way the Moon's hot mantle flowed.
The South Pole-Aitken basin is an immense basin in the southern part of the lunar far side. It is over half the width of the United States. The impact must have been apocalyptic. It is one of the largest impact basins in the solar system.
The scientists wondered if the event could be related to the chemical difference. Around the time the maria started repaving the lunar near side, the impact occurred around 4.3 billion years ago. The scientists focused their attention on the hot fluid rock beneath the Moon's mantle, because the material brought up in the maria volcanism came from there.
They used models of heat flow through the Moon to model different scenarios after the impact. The impact generated a huge amount of heat that moved through the Moon's interior, changing the way the mantle flowed.
A huge pulse of heat would have moved to the opposite side of the Moon. KREEP material is associated with radioactive elements such as thorium and uranium. This would have caused a lot of volcanism. The most obvious feature of the full Moon is the Oceanus Procellarum, a large mare in the lunar northwest. This region has a lot more thorium and titanium, which is associated with KREEP.
About 3.8 billion years ago, another huge impact blasted out Mare Imbrium, one of the last of the giant impacts on the Moon. That also let loose a lot of KREEPy stuff from under the surface.
The South Pole-Aitken basin is big enough to affect the entire Moon, so this is still hypothetical, but the science fits, and I have to say it doesn't involve any special circumstances. Scientists like hypotheses that use what we already know and don't ask for any one-off occurrences. The hypothesis is not certain about the difference in thickness, though it is possible that other physics was involved.
During the Apollo era exploration of the Moon, a lot of this evidence, like KREEP minerals, was discovered. We don't really understand the origin and evolution of the Moon, but we are about to embark on a new era of lunar exploration. What else are we going to learn?