If astronomers were ever to discover that there is enough matter in the universe to cause its current expansion to slow down and stop, we would be living in a universe destined to collapse in the far future. Towards the end of this collapse, galaxies will collide and shred apart, and later on, even individual stars would collide so that the universe would become a hot dense gas at temperatures over a billion degrees. Cosmologists call this the 'Big Crunch' to distinguish it from the Big Bang. It will only happen if there is enough mass in the universe for the gravity of all its parts to overcome the current expansion speeds of all the galaxies. It does not seem that this is very likely since astronomers can only find about 10 percent of the mass needed to cause the universe to collapse. At any rate, this would not happen for another 30 billion years or so.