All mass in the universe is gravitationally attracted to all other mass in the universe. "Gravity" exists everywhere.That said, the effects of gravity can be canceled out. If gravity pulls you one way, something else can pull the other way and cause an effective "weightlessness" as if there was no gravity.To analyze this question easier, let us neglect all the other celestial bodies in the universe (like the sun, other stars, planets,), and only deal with the Earth.In this case, if you Earth were a uniform, hollow, spherical shell, then ANYWHERE inside, the net gravitational force would be zero...in effect there would be no "gravity" / gravitational force.The math which goes into proving this fact is beyond the scope of this post, but if you were anywhere inside a uniform spherical shell...there would be no net gravitational attract because of the symmetry of the system.Think of it like this...If you were in the exact center of the sphere, I think it is pretty easy to conceptualize the fact that all the Earth's mass would be pulling you equally in all directions because you are in the center, thus no gravity. But it is also true if you are not in the center, but rather closer to one 'side' of the sphere than the other. If you are closer to the shell, you are that much closer to some mass which will pull on you...but the amount of mass which pulls on you is less than if you were in the center, the other mass is pulling the opposite direction so the net effect is zero.You might recognize this effect is similar to that of electric fields inside charged conducting spherical shells if you have studied Gauss's law.If we then take the other celestial bodies into consideration again, then there would be a net gravitational force…the same force which keeps you and the Earth in orbit around the sun and the rest of the galaxy.http://en.wikipedia.org/wiki/Shell_theoremhttp://www.kineticbooks.com/physics/trialpse/13_Gravity%20and%20Orbits/06/sp.htmlThe second link, above, offers an excellent mathematical derivation / proof of this concept...but in order to see the page you first have to click on the "continue on" link if you don’t want to install their software (which is not required).