The term "dark side of the moon" is a bit of a misnomer. In reality, there is no permanent dark side of the moon. The moon is tidally locked with Earth, which means that it takes the same amount of time for the moon to complete one rotation on its axis as it does to orbit the Earth. As a result, the same side of the moon always faces the Earth, while the other side is often referred to as the "far side" of the moon.
The far side of the moon is not always dark, as it experiences day and night just like the side that faces Earth. The reason it is often associated with darkness is that from Earth, we never see the far side directly because it is always facing away from us.
As for why we haven't sent crewed missions to the far side of the moon, there are a few reasons. First, the logistical challenges of sending missions to the far side are more complex compared to missions to the near side. Communication with spacecraft on the far side is more difficult because the moon itself blocks direct radio signals. To overcome this, relay satellites have been used to establish communication links.
Secondly, the majority of lunar exploration has focused on the near side because it has unique geological features, such as the large and easily accessible basins that are of interest to scientists. The near side also provides more favorable conditions for communication with Earth, as it allows for direct line-of-sight communication without the need for relay satellites.
However, exploring the far side of the moon has gained increasing interest in recent years. In 2019, China's Chang'e 4 mission successfully landed on the far side, making it the first mission to accomplish a soft landing and rover exploration there. This mission has provided valuable insights into the geology and composition of the far side.
In the future, as space exploration continues to advance, there may be missions planned to further explore and study the far side of the moon. These missions could contribute to our understanding of the moon's history, geology, and potentially serve as a stepping stone for future deep space exploration.