Today's Universe is known to have well-defined structure on many scales. On cosmological scales, those involving the global structure and evolution of the Universe, the fundamental element are the galaxies, collections of stars, gas, dust and dark matter, that are present in a finite number of morphologies - elliptical, spiral and irregular - and dimensions. With an average content of 100 billion stars and a total mass up to a thousand billion times the mass of the Sun, the galaxies represent the fundamental building blocks from which the large-scale structures in the Universe are built. The density of galaxies is extremely high near the rare density enhancements, the galaxy clusters, connected by filaments and "sheets" composed themselves of galaxies. This topology is somewhat like that of a sponge, with a large fraction of the space essentially devoid of galaxies.
How the Universe evolved from the initial condition of almost perfect homogeneity, as testified by the observations of the cosmic microwave background (CMB), to this very high level of structure, and the physical processes involved, have been the central themes in the fields of observational a theoretical cosmology since the '60s. Despite significant progress, especially in the last 30 years, various aspects still lack a clear physical understanding and new and deeper questions have actually arisen as a result of the more recent observations. These questions push the imagination and efforts of astrophysicists towards new frontiers and are the motor of modern astrophysical research. In fact, we find ourselves in the lucky position of having, for the first time, a "standard cosmological model" able to explain virtually all the observations. Amongst these, not only the large scale distribution of galaxies, that today we have reconstructed up to scales of hundreds of millions of light years, but also the ever more precise observations of the cosmic microwave background and the luminosities of high redshift supernovae.
In this model, that works surprisingly well, however, the mass/energy density of the Universe is dominated by two "dark" components, or "substances", that do not emit light or any form of electromagnetic radiation. Of the total budget, approximately 5% is "normal" material (that with which we are familiar, so-called "baryonic"), and about 20% is "dark matter", that we can't see but that we detect via its gravitational effects on galaxies and larger structures. The remaining 75% is made up of another kind of "dark energy", necessary to explain the expansion of the Universe during the last 7-8 billion years. As was shown only in 1998, using measurements of the luminosity of type Ia supernovae in galaxies at high redshift, the Universe today appears to be in a phase of accelerated expansion, pushed by the repulsive force of this dark energy. In this scenario the large scale cosmic structure that we observe is the result of the combined effects of the global expansion and the gravitational amplification of small primordial mass/energy density perturbations. The origin of these perturbations can be traced to quantum fluctuations in the first fractions of a second of the life of the Universe.
Italian astrophysical research has earned a leading role in this field during the last 15 years, achieving fundamental results and driving new large experiments that will bear fruit in coming years.