Necro: Dark matter was theorised to exist based on observations of gravitational anomalies and the general behaviour and structure of objects in the known universe. In order for galaxies to move the way they do, there would have to be a hell of a lot more matter out there to exert the observed gravitational effects - in the order of 90% more than has been observed. Following this theory, if there is enough dark matter, the gravitational effect it exerts could be enough to stop, or even reverse the expansion of the universe (Big Crunch and possibly another Big Bang).
It turns out that most of it is present in the form of intestellar gases.
As I suggested to PK earlier, get hold of a copy of 'A Brief History of Time'. Stephen Hawking is excellent at explaining this sort of thing.