Fact Sheet
Lighting up the Darkness
Elusive "dark matter" shows itself through its gravitational influence on a large cluster of galaxies in this composite image from NASA. The image shows hundreds of galaxies, plus the X-ray glow of hot gas (blue). But most of the gravity of the cluster comes from the region shown in pink. There is no actual "glow" of any kind from this region (the pink was inserted only as a graphical representation), so astronomers conclude that it must contain large amounts of dark matter. No one has yet determined what constitutes dark matter, but dark matter and dark energy make up about 96 percent of the known matter and energy in the universe. One McDonald Observatory project will try to measure dark energy. [Credit: NASA/CXC/CfA/M.Markevitch et al/STScI/ Magellan/U.Arizona/D.Clowe et al/ESO WFI]
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