Matter can neither be created nor destroyed. Therefore, the amount of matter in the universe now is the same amount as was around directly after the big bang.
Matter attracts itself to other matter through gravitational pull. Therefore, the natural tendency of matter is to join together with another mass.
So, given that there was enough force during the Big Bang to break apart the most powerful gravitational attraction ever (i.e. all the mass of the universe), why would there in the future be enough of a gravitational pull to bring them all back together, when galaxies are now so far apart from one another?
In other words, if there is enough mass in the universe to bring about a Big Crunch, how did the Big Bang ever occur in the first place? How did matter escape the pull of all other matter?