Invisible dark matter forms a hazy radial structure in this composite image of a galaxy cluster from Hubble Space Telescope. Although the dark matter itself produces no detectable energy, astronomers can map its presence by measuring how it affects the light of the galaxies beyond it. This image shows the galaxy cluster Cl 0024+17. Dark matter appears to make up about 85 percent of all the matter in the universe, although scientists are still trying to deduce its nature. [NASA/ESA/M.J. Lee (Johns Hopkins)]