Most scientists believe that the universe is not headed for a Big Crunch. Both the average density of the universe and the critical density change with time. When the universe was very young, and very dense, these numbers changed very rapidly. To make the Standard Big Bang theory correspond to reality, cosmologists had to make the assumption that the average density of the universe was equal to the density immediately following the Big Bang. This assumption, like the isotropy assumption, isn't explained.
Inflation comes to the rescue again.
Inflation's rapid expansion caused space to become flatter, forcing omega toward one, no matter what its initial value. Although the estimated values of Omega hover around one, the range implies a lot of uncertainty about the average density of the universe. We just don't know how much dark matter is out there.