We call it "dark" because we are sloppy.

The same side of the moon always faces the Earth. If the same side of the Earth always faced the moon, then only one hemisphere would be able to see the moon. Since you can see the moon from everywhere on Earth (not at the same time...), we know that the same side of the Earth does not always face the moon.

Unless you're thinking of the "outside" of the Earth. /s