About 96% of the energy density of the Universe exists in the form of dark matter and dark energy, which govern the structure and evolution of the Universe on the largest possible scales. Clusters of galaxies are the largest collapsed objects in the Universe. Their formation and evolution is dominated by gravity, i.e., dark matter, while their large scale distribution and number density depends on the geometry of the Universe, i.e., dark energy. They are filled with hot baryonic gas, which is enriched with elements by star formation and stellar explosions, and is preferably detected by its high energy radiation. X-ray observations of clusters provide information on the dark matter and dark energy content of the Universe, on the amplitude of primordial density, on the complex physics governing the formation and evolution of structures in the Universe and on the history of metal synthesis. Whilst nearby clusters of galaxies have been studied at great detail with existing X-ray satellites, nothing is known about their formation and evolution in the early Universe. In addition, the fate of almost 50% of the baryons in the Universe, believed to reside in warm/hot filamentary structures observable with X-ray absorption spectroscopy, is still a mystery. To study the genesis of groups and clusters of galaxies and the cosmic web at up to z ~ 2, and the evolution of the physical state and chemical abundances of the intergalactic medium, an X-ray telescope combining a very large collecting power with excellent energy resolution and good spatial resolution is necessary.
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