If the universe is expanding, will this expansion continue forever? We know that the only force capable of stopping this expansion is gravity. So what happens to the universe depends on two things: 1) How fast that universe was hurled outward during the BB, and 2) the strength of the gravitational pull. We can check the first by refining the red shift measurements, especially that of distant galaxies. The second depends on how much matter there is in the universe: its mass. 

If there is enough mass to generate enough gravity to overcome the expansion, the universe will stop, reverse its course, rush inward to form a new singularity (the Big Crunch). Such an oscillating universe is called a closed universe. If there is not enough mass, then the universe is open, on a one way trip; expansion will continue forever, and the universe will die from entropy as all the objects in it burn out (the Big Chill). The third possibility lies between those two, namely that the universe continues to expand but slows and eventually comes to a stop in an infinite future, but at that point the galaxies will be so far from each other that they will not come back together. Such a universe is called flat, or critical, and will also die an entropic death 

So how much matter is there in the universe? If we count up all the matter that gives off light and that we can observe with telescopes, we do not even come close to having critical mass, and there is no question that our universe is open. However, careful observations of the behavior of stars in galaxies has shown that there is a lot more gravity and therefore mass than we can detect visually, and therefore, there must be much more matter than we can actually see. We call this matter dark matter because it cannot be seen. We know there is more dark matter than visible matter, and many people maintain that there is enough dark matter for the universe to be critical, and perhaps even enough for it to be closed. The answer is that we are not sure, and measuring how much mass there is in the universe is a critical topic in astronomy today.