Black holes. Composite image of a collection of active supermassive black holes (dots) inside the hearts of galaxies, with their colours representing different energies of X-ray light - red being the lowest energy, through yellow and green, to blue as the highest energy. It is thought that some black holes produce more high-energy X-ray light than others due them more actively feeding off surrounding clouds of dust and gas, but also because the material surrounding them may be so dense that only high-energy X-ray photons can penetrate the thick screen. Image composed of high-energy X-ray data from NASA's Nuclear Spectroscopic Telescope Array (NuSTAR, blue) and lower-energy X-ray data from the Chandra X-ray Observatory (red, yellow and