The moon's rotation is the same as its orbital period, thus it always has the same face toward the earth. The issue of whether the moon is 'natural' aside, any object with less mass than that of the earth can remain in orbit indefinitely if it has sufficient velocity to counter earth's gravitation.  In the world of planetary physics, there is no 'too big'; there is only a balance of velocity and mass. There are a number of popular theories as to the moon's origin, but nothing has been proven or settled.  There are three mainstream theories: The moon was formed along with the earth; the moon was an object that was captured by earth's gravity; the moon was broken off from earth when the earth collided with some large object. Of course there are fringe theories such as that the moon is a spaceship, the moon is a hologram, or the moon is made of green cheese; all equally likely.  LOL!