Big Crunch?

What will be the ultimate fate of the universe? According to the big bang model, if there is sufficient matter in the universe, gravitational forces will halt the expanding universe, and eventually the universe will start to collapse, ending up in a big crunch. However, if there is insufficient matter, the universe will expand forever, and the universe will eventually cool off completely and die a slow heat death.

  Although of course exceedingly difficult, astronomers have attempted to measure the total mass of the universe, and found that it was less than that required for the big crunch to take place. However, there may be what is called dark matter in the universe which may provide the necessary mass to halt the expansion. This dark matter, by its nature, is difficult to detect as it cannot be seen, and so indirect measurements of its presence have been made by inferring its influence on visible objects.