Perhaps the Traveler put a super dense material at the core as part of the terraforming process? Here is something to wake the Hive, on the subject of the Moon. Have also seen the argument about the Moon not having an atmosphere. Look towards the horizons, next time you are there. There is definitely some sort of atmosphere, there. It is thin, but you can definitely see the bluish coloration. Don't care if it is all based on real science or not. Games are not exactly known for scientific accuracy.