The ET will take us back into the universe’s past, to the dark age following the big bang, and forward into its future, helping to find an interpretation of what its fate may be. The ET will foster the understanding of what the universe is made of and the astrophysical mechanisms of its most violent events. Indeed, little is known about the universe. One knows just about 5% of it: the ordinary matter which we and all that we can currently observe are made of. In contrast, we know virtually nothing about all the rest, approximately 95% of the universe, except that 25% consists of a different form of matter, called dark matter, and 70% consists of a mysterious dark energy. The nature of both is unknown. Yet one does know that they played a decisive role in the formation of the universe and that they will play an equally decisive role in shaping its future. Perhaps the universe will expand with increasing speed, condemning us to darkness and thermal death: matter will decay, even black holes will evaporate. Or perhaps the acceleration will be so violent that it will tear spacetime apart, destroying all the structures of the universe. Alternatively, the universe might slow its expansion, then contract, returning to the microscopic size from which it began, in a backward big bang. Perhaps the process could repeat itself in an endless series of cycles. At the moment, one does not know: the ignorance about what the universe contains inevitably reflects on the current inability to predict its fate. Yet the Einstein Telescope could help pave the way toward understanding the nature of dark energy, and dark matter, by investigating primordial black holes, axion clouds, and dark matter accumulations around compact objects. The ET will also probe physics near the event horizon of black holes, testing general relativity under extreme conditions and testing new theories such as quantum gravity. It will be able to test possible changes of general relativity on a cosmological scale. In addition, it will study the entire population of stellar black holes, and those of intermediate masses accessible throughout the history of the universe, enabling us to understand their origin (stellar versus primordial), evolution and demography. The ET will observe the inspiral phase of neutron stars and may provide unprecedented insight into the internal structure of neutron stars, probing fundamental properties of matter in a completely unexplored regime, such as quantum chromodynamics under ultra-density conditions, and possible exotic states of matter. The ET will eventually open the doors of a universe yet to be discovered to the scientific community.