Gravity is not something that exists like a substance, it is a force which is exerted on everything with mass in the Universe. Following from that yes, the total mass in the Universe is constant but if the Universe were made up from many similarly sized objects and nothing else then your theory would hold (probably) but the point is that things collide, masses join and supermassive stars collapse creating black holes. These black holes (which have effectively infinite mass at their centre I believe) therefore attract everything in the area towards them and 'swallows' it up. I guess in the case of two black holes, the larger one will win...
RE: Neutrino>Usain Bolt: It's the way the experiment is carried out which doesn't stand up for me though. I'm quite happy to accept that something may be able to go faster than light, hell why not? However, this experiment relies on CERN being able to tell exactly when an individual neutrino leaves their collider in order to determine when it arrives in Italy, correct to billionths of a second. They aren't even measuring the same particles but are using some convoluted probability to determine what they're measuring. I think in this instance no one will ever be sure.