Supdon3
Kal-El
I think that the theory is that all the mass existing in the universe has some amount of gravitational pull and all of it working on each other would provide enough of a pull on all the other mass that eventually the expansion would slow and stop and then pull in on itself. There is no longer any force pushing mass outward, its working on inertia so the gravitational pull of all the mass in the universe could conceivably, no matter how small eventually slow down the expansion till it stops and reverses because of simple physics, an object in motion will remain in motion unless acted upon by another force.