I know this will sound crazy but I just wanted to put it out there: Maybe the earth is (or has been) expanding like a balloon, but not gaining much mass at all. The earth could be hollow on the inside, as well as many other planets and moons, and maybe even stars. Our understanding and calculations of gravity is based on the assumption that these bodies are solid to the core. I believe that empty space has density directly proportional to the volume of matter around it(space is pressurized from the existence of matter). I think it's possible that the space on the inside of our planet pushed outwards, as our planet moved into a less dense area of space. This would be similar to the way a filled balloon would expand as it enters an area of lower pressure.