An image of the bullet cluster taken by NASA's Chandra X-ray telescope. Scientists claimed the image was proof of normal matter (pink) being separated from dark matter (blue). Another group now says the observations can be explained by a modified gravity of theory, and that dark matter is unnecessary. Credit: X-ray: NASA/CXC/CfA/M.Markevitch et al.; Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al.; Lensing Map: NASA/STScI; ESO WFI; Magellan/U.Arizona/D.Clowe et al.
