THE EXPANSION OF THE UNIVERSE IS ACCELERATING
http://luth2.obspm.fr/~etu/fay/en/acceleration.html
http://en.wikipedia.org/wiki/Accelerating_universe
http://imagine.gsfc.nasa.gov/docs/sc...rk_energy.html
http://www.astro.umd.edu/~immler/Lecture_13.pdf
Not only is there dark matter, there is dark matter. The relative concentrations of dark matter, dark energy, and matter/energy have been in flux for the history of the universe.
To picture the expansion, think of bread dough you put into an inifinite oven. The dough can expand to fill as much space as it needs. Or, finite stuff expanding into an infinite space.