It is not logical to think that the earth is hollow. But the enormous pressure at the earth's center may cause the rock to be in a state that we don't understand. It may be able to move and flow as a super dense liquid, or semi-solid gas of some sort. Gravity from an object in space is directly proportional to it's mass. The gravity on the moon is much less than the gravity on earth because the moon is smaller, and weighs less than the earth. The center of gravity between the earth and moon, would be closer to the moon because the moon has less gravitational pull the farther away from the surface you go while the strength of the earth's gravity is relatively strong.The "center" or region of space between the earth/moon system where the gravitational pull from both planets is equal, would be a far distance from the surface of the earth, and a closer distance to the surface of the moon. At this "center of gravity" the pull from the earth would equal the pull from the moon. If you pass the "center" in space, the pull from the moon becomes stronger on a spaceship while the spaceship feels less effect from the weaker pull of the earth.