It's incredibility fascinating isnt it, and frustrating at the same time. There are so many theories and so many unanswered questions.
Getting back to what I think
was the main question in this tread, re. the density of the universe, it's another one of those (as yet) unanswered questions. From what I've read it's not so much a mass problem as even dark matter will still leave us 60% or so short, but a vacuum energy density question, re dark energy.
Still hairy fairy when we still don't really know whether the universe is open, closed or flat.