No One Knows What Causes Gravity

It is generally accepted that gravity is a force where mass attracts mass. But this is merely an assumption. While we know that mass is attracted by gravity, we have no proof that it is mass that causes gravity. And we do not even know what mass is, besides being a property of most forms of matter, so even if gravity is caused by mass we do not know what it is.

Gravity may just as well be caused by the capacitance inherent in most types of matter, in which case we would still get Newtonian equations for orbits and change in gravity with altitude, but end up with the possibility that Earth and other large bodies are hollow at their core.

Sir Isaac Newton 

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