A small second moon may once have orbited Earth until it slammed into the other one, enabling two distinct sides, a new study says. 

The second moon would have been about 750 miles wide and may have been created by the same collision between the planet and a huge object that scientists think helped create our moon, astronomers said. The theory was outlined Wednesday in the journal Nature.

The gravitational tug of war between the Earth and the moon decreased the speed at which it turns, causing us to be able to see just one side.

The "dark side" of the moon was first seen by the Soviet Luna 3 spacecraft. 

Widespread plans of volcanic rock, or "maria," fill the near hemisphere while the far one only has a few.  Also the near side's surface tends to be low and flat while the far side is mountainous.

Computer simulations reveal that a second moon attached itself to the first moon, causing vast differences in our moon's appearance.