Through the long-range force of gravity, mass controls the evolution of the universe. Studies of the internal motion of large, gravitationally bound systems such as galaxies and clusters of galaxies have already shown us, rather convincingly, that the mass in these systems is dominated by some form of matter that is not luminous at any wavelength from radio to x ray. (See the article by Scott Tremaine in PHYSICS TODAY, February, page 28.)
The total mass of this "dark matter" exceeds that attributable to luminous matter - stars, gas and dust - by at least a factor of ten. Even without surveying the vast space between clusters of galaxies, astronomers have already concluded that there is enough dark matter within the clusters to add up to 20% of the critical "closure" density c, given by 3H02 / 8G, where G is the gravitational constant and H0 is the present Hubble constant. Unless the present mean mass density of the universe exceeds c, the Hubble expansion will continue forever. The current fashion in cosmology is to suppose that inflation in the first moments after the Big Bang has brought us almost precisely to the closure density.
Until recently, all the evidence that dark matter dominates the mass of large stellar systems has come from dynamical observations: redshift measurements of the line-of-sight velocities of galaxies moving within clusters and of stars and gas clouds orbiting within galaxies. Applying energy conservation and the virial theorem (which assumes that statistical equilibrium has been reached) to such a gravitationally bound system, one always finds that the total mass any distance from its center is much larger than the stars and gas clouds can account for. In galaxies and clusters, the observed dispersion of velocities about the mean is so large compared with what one would expect from assigning one solar mass per solar luminosity that one might think these systems are not gravitationally bound. Thus unseen matter has to be invoked just to provide gravitational binding against the rapid internal motion we do see.