So in addition to the gravitational attraction of the galaxies in the cluster, there also was the effect on light passing through one. So if you put in all of the dark matter, you're missing sort of a factor of ten in closing the universe. But the observations put us within a factor of ten now, and therefore within some very tiny fraction, back at the earliest time we can think about. Science now tells us that our universe was critically adjusted at the beginning. Inflation is the name of one theory that physicists use to explain how the universe was launched at the critical density, though it doesn't explain why such a perfect balance among physical constants should have permitted inflation in the first place.
All cosmologists agree that many other critical parameters remain a mystery.