For decades, scientists have been trying to understand why the Earth-facing side of the moon looks so different from the moon's far side. Now, they may have an answer to that mystery.

"I remember the first time I saw a globe of the moon as a boy, being struck by how different the farside looks," Jason Wright, an assistant professor of astrophysics at Pennsylvania State University, said in a written statement. "It was all mountains and craters. Where were the maria? It turns out it's been a mystery since the fifties."

The lunar maria or "seas" are massive plains created by ancient volcanic activity. They appear as the dark splotches on the lunar surface.

Whereas the near side of the moon is filled with these plains, the far side -- which was first observed in images captured by Soviet spacecraft Luna 3 in 1959 -- features few maria.

But why?

As researchers at Penn State now theorize, the lack of maria has to do with the thickness of the crust on the two sides of the moon: the crust on the far side is much thicker than that on the near side.

After the impact, both the Earth and the newly formed moon were incredibly hot. The near side of the moon cooled gradually because it bore the brunt of the heat radiating from the Earth. The moon's far side cooled more quickly, which allowed for the development of a thick crust chock-full of aluminum and calcium, according to the new theory. When lunar magma subsequently spewed forth to create the lunar maria, the far side's thick crust meant it was less affected than the near side.

The bottom line? It's that thick crust -- and the resulting lack of maria -- that explains why the two halves of the moon look so different.