I once heard that the moon was hollow, I would like to know if this is true and why?
The Moon is not hollow. It is possible to tell this because when we have had things in orbit around the Moon, how they orbit in the Moon's gravity tells us about the mass of the Moon. We also know the size of the Moon so we can work out the density and see that it is not possible for the Moon to be hollow.
Karen was a graduate student at Cornell from 2000-2005. She went on to work as a researcher in galaxy redshift surveys at Harvard University, and is now on the Faculty at the University of Portsmouth back in her home country of the UK. Her research lately has focused on using the morphology of galaxies to give clues to their formation and evolution. She is the Project Scientist for the Galaxy Zoo project.