The large dark patches are called Maria (Latinfor sea), and are large lava plains. They were made by the impacts of large asteroids and comets billions of years ago that made huge impact basins - like, really large craters. This allowed magma from the interior to reach the surface and fill those basins with lava. The reason why they are mostly on the Earth-facing side of the moon is still not fully understood.
We know that large objects would have hit the moon randomly, and indeed the largest impact basins is on the far side. (Google the South Pole-Aiken Basin!) so the question is really why did the basins on the near side fill with lava, but not the ones on the far side? Many planetary scientists believe themoons’ crust on the near side is thinner than the far side, making it easier for lava to make it to the surface.
It may also be that the chemical composition of the magma is slightly different there, making it easier to flow through fractures in the impact basins. But scientists are still not sure, and it’s still a matter of debate and investigation.
- Professional astronomer on Reddit.
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