Using the Hubble Space Telescope, astronomers are observing galaxy clusters-- some of the oldest and largest structures in the universe-- to work out the location of clumps of dark matter.
Dark matter can only be detected by noting its gravitational influence on normal matter, and on light. Astronomers determine its existence within and around galaxy clusters, for example, by measuring the visible mass in the cluster. If there's not enough mass there to account for the behavior of the components and the overall structure of the cluster, they deduce dark matter is making up the difference. Gravity also bends light. If light from a more distant source, passing through a cluster, is warped more than it should be given the visible mass, dark matter is again assumed to be the cause of the warping.
Theoretical physicists say perhaps 98 percent of the matter in the universe is dark. Everything we see amounts to froth.
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