The dark spots on the moon are called "maria" ("mare" in the singular), which literally means "seas" in Latin. However, despite the misnomer, the maria are low-lying areas that are more like flat plains, that are devoid of water.
The maria mainly exist on the near side of the moon, the side of the moon that always faces the Earth. On the far side of the moon, hardly any maria exist. In total about 17% of the moon's surface is covered by maria.
In contrast to the maria are the light-colored lunar highlands, which dominate the far side of the moon and are older than the maria. Craters abound in the highlands, but the maria contain almost no craters. An examination of specimens from the maria and from the highlands shows that rocks from the maria are only about 3.5 billion years old, but rocks from the highlands are about 4.5 billion years old.