What happened when the moon 'turned itself inside out' billions of years ago? 

 The moon underwent a reversal around 4.2 billion years ago, flipping itself "inside out" after a giant impact to create the picture of the faithful lunar companion that we see today. 

https://cdn.mos.cms.futurecdn.net/XmfNHMkYpU9gwqqfqKKF57.png

https://www.space.com/moon-inside-out-billions-years-ago