The X-ray satellite will reveal the hot gas which dominates the baryonic (visible) matter content of the Universe today, contained in galaxy clusters, galaxy groups, and intergalactic filaments. It will also show how these structures formed and evolved, tracking the thermodynamic, chemical and dynamical evolution of these structures through cosmic time. Athena will explore when and how elements were created and dispersed into the intergalactic medium. Peering through dust and obscuring clouds of gas, Athena will discover and map supermassive black holes throughout cosmic time back to when the Universe was still assembling galaxies. These images and spectra will uncover the history and evolution of ordinary, visible matter, as well as their interplay during the formation of the largest structures. Athena's observations of neutron stars will show how matter reforms under crushing pressures well beyond any laboratory, while studies of spinning black holes will reveal how these objects form and grow. In addition to these core goals, as an observatory with exceptional capabilities compared to current facilities, Athena will also open up a huge discovery space sure to yield new phenomena we cannot currently envisage.