The most crowded collision of galaxy clusters has been identified by combining information from NASA's Chandra X-ray Observatory, Hubble Space Telescope and the Keck Observatory on Mauna Kea, Hawaii. The system MACSJ0717.5+3745 (or MACSJ0717 for short) is located about 5.4 billion light years from Earth. Credit: NASA/CXC/IfA/STScI/C. Ma et al.
