Current observations say that there is NOT enough matter in the universe to reverse the expansion and head us towards a "Big Crunch" (closed universe). There certainly isn't enough "visible" matter (stars and gas and stuff), but there is evidence that there is matter that we can't see, so called "Dark Matter". Current measurements give a value for "Omega" (the density of the universe divided by the density required to halt the expansion) of 10 - 40% (including dark matter). Many theorists still prefer an Omega of one, but if the matter is out there, we haven't seen it yet.