Scientists believe that the universe consists mainly of dark matter -- that is, matter that is essentially invisible because it does not emit radiation.
Since dark matter does not appear in images, its existence has been inferred through unusual means. One of the earliest indications arose from analysis of the motion of astronomical objects.
"There's long been evidence that the stars in the galaxy were moving in ways that suggested there was more matter than we had observed," Astronomy Dept. Chair Robert Rood said. "If you look at the motion of stars oscillating above and below the galactic plane, there has to be more gravity, more force in the spring, than is produced by visible matter because of how fast they are moving."
Examination of the rotation of galaxies has provided further evidence supporting the existence of dark matter. Because visible mass is concentrated in the center of galaxies, the rotational velocity of stars should decrease moving toward the outside -- however, it remains constant, indicating that non-visible mass must be present in the outer regions. Similarly, many galactic clusters do not contain enough visible mass to remain gravitationally bound; in some the visible matter constitutes as little as 1/300th of the total.
The density of mass in the universe will ultimately determine its destiny. An open universe, with a low density, would continue to expand forever, while a closed universe, with a high density, would eventually reverse its expansion and collapse in a reverse big bang. In the intermediate case in which the mass per unit volume density, called Omega, is equal to one, the universe is flat and will attain a constant size.
By examining the cosmic background radiation, the Wilkinson Microwave Anisotropy Probe has mapped light from the beginning of the universe, giving insight into its structure.
"WMAP has shown pretty conclusively that the universe is flat," Rood said. "A plausibility argument is that, by the observed rate of expansion, Omega is at least .1, but it must be less than two or expansion would have already reversed."
An analysis based on nucleosynthesis
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