@Al
Now you've raised the dark matter question.

It certainly appears that the universe's expansion is accelerating. But the rate of expansion is inconsistent with the amount of known matter. It's believed that dark matter (which may be some form of exotic matter that we can't detect, or may exist in parallel dimensions) is causing gravitational effects which would explain this discrepancy.

It's a hot topic in cosmology.

Edit - the Big Depressing Cold end is when the universe expands so much that all its energy is dissipated so widely it cools down and fizzles out. Whereas if there's enough matter, and hence enough gravity, it'll all come together again in a big hot crunch.
( K2k6 has a proper job these days, Thu 16 Oct 2008, 10:23, Reply)