the moon was volcanic at one time before the earth trapped it or it was placed in orbit long long ago.
the gravity from the earth attracted its mass and due to this attraction slowed its rotation through a offbalance which caused a cooling of the moons insides from lack of friction.
the heavier parts that were basicaly liquid moved towards the earth and as time went by the moon became a cold solid object.
its density is offbalance and this denser part always attracts the earth more than the less denser part.