Bruce Murray Space Image LibraryChandra Deep Field South Chandra Deep Field South This image taken by NASA's Chandra X-Ray Observatory showcases the highest concentration of black holes ever seen, equivalent to 5,000 black holes over the area of the sky covered by the full Moon. It took over 7 million seconds of Chandra observing time to create, and at the time of its release in 2017, it was the deepest X-ray space image ever obtained. The colors in this image represent different levels of X-ray energy detected by Chandra. Here the lowest-energy X-rays are red, the medium band is green, and the highest-energy X-rays observed by Chandra are blue. NASA / CXC / Penn State / B.Luo et al.