According to research analyzed by NASA's Gravity Recovery and Interior Laboratory (GRAIL) mission, the reason for the difference is because the Moon's crust is thinner on the near side compared to the far side. The dark splotches that make up the large lunar maria are lava-filled impact basins that were created by asteroid impacts about four billion years ago.
This discrepancy is explained by research conducted by NASA's Gravity Recovery and Interior Laboratory (GRAIL) mission, which indicates that the Moon's crust is thinner on the near side compared to the far side. The prominent dark spots on the lunar surface are actually large impact basins filled with lava that were formed around four billion years ago.