What if: Gravity is a pushing force, not a pulling force. Matter is actually lower pressure space, while a vacuum is higher pressure space. This idea actually makes a lot more sense than gravity somehow reaching out from a mass and pulling another object in. So imagine two objects in space... same size and shape as the earth and the moon, and spaced the average distance apart that the earth and moon are normally spaced. Now forget about any velocities and imagine that they are both starting still at the beginning of this experiment. Now according to known physics, the earth-sized object would creep very slowly towards the moon as the moon's inferior gravitational pull would still exert a small effect on the more massive object. Also, the moon-like object would move significantly faster towards the earth-like object because the bigger object would have a greater pull on the smaller one. Now this scenario makes sense, expect for one thing- nobody understands the mechanism which causes these two objects to reach out and grab the other one towards it. Nobody knows what gravity really is. So, imagine this instead... think of the earth and moon as relatively hollow objects, and the surrounding space as a more dense material. The moon object would be pushed towards the earth due to the lower pressure area on the side of the moon facing the earth, and the earth object would be pushed towards the moon slightly because the moon creates a (smaller)low-pressure area as well. In this scenario, the objects would still behave exactly the same as they would in classic gravity models except that the force of gravity is no longer a mystery force. What we perceive as a pulling gravity is really just lower-pressure areas trying to be filled in. I imagine this idea could be demonstrated in a zero-gravity environment. A tank could be filled up with water representing empty space, and tiny air bubbles of various sizes could be introduced into the water one at a time. I imagine that bubbles far enough apart would have not move, but two bubbles introduced close enough to each-other(within their "gravitational fields" so to speak) would start to "gravitate" towards each-other. This is because two air bubbles close enough to each-other would each experience a greater inward pressure on the side away from the other bubble.