Exploring the early universe with current and future X-ray missions

Thanks to previous ROSAT deep surveys and recent observations with Chandra and XMM-Newton the bulk of the background at energies 0.1-10 keV could be resolved into discrete sources, most of which were identified as active galactic nuclei, often with large absorption columns. The X-ray background is thus the obscured "echo" of the formation and growth of the supermassive black holes we observe as dormant dark remnants in the centers of nearby galaxies. The future studies will focus on a better understanding of the formation and accretion history of the black holes as well as their physical environment.