Brant,

In stellar metamorphosis theory the Earth and all stars cool and form iron cores as the iron has the lowest ionization potential. The iron moves to the center because of its magnetic properties and the fact that iron as a plasma has a very low ionization potential, which means its the first to move from a plasma to a gas to a solid. Since it is the first to cool, and is magnetic with electrical current (iron becomes an electromagnet) it is the first element to clump together in the centers of stars. Thus, stars are only hollow when they are young and still ionized. Young stars that have not cooled and solidifed are hollow, not ancient ones like Mars, Mercury, Venus, Neptune, Uranus, Earth, Moon, Ganymede, Titan, etc.

I am not saying that the Earth isn't hollow right now, I am saying it used to be when it was fully ionized and resembled the Sun and other much younger stars.