Abstract
The great structures of the Universe are essentially made of invisible dark matter, but galaxies are associated with them and serve as a tracer. The mapping of the Universe in three dimensions (the3rd dimension, depth, is given by velocities, given the expansion of the Universe) has accelerated considerably in recent years. Our knowledge is growing exponentially. While at the beginning of the 20th century, up until 1920, astronomers wondered whether there were galaxies, worlds apart, outside the Milky Way, the expansion of the Universe was established around 1930, and gradually maps of galaxies in filaments were obtained. The CfA-2survey carried out from 1985 to 1995 obtained 18, ,000 galaxy spectra in ten years. Then it was dethroned by the Sloan SDSS, with a million galaxy spectra, and it's still going ! Progress is due to the greater performance of telescopes, but above all to multiplexing, using fibers or lenses, which enables 400 to 1 000 spectra to be recordedsimultaneously. These large surveys on dedicated telescopes have revealed even larger structures, such as the SDSS Great Wall of 1 370 Mpcsize ! Large surveys can also be used to collect stars in the Milky Way, local galaxies and the 300 billion galaxies visible up to our horizon, high-redshiftquasars , etc. At big z, it's the deep surveys of the Hubble Space Telescope (HST) that have enabled us to learn more about the distant Universe and galaxy formation. For nearby galaxies, where particular velocities dominate the expansion of the Universe, it was necessary to use a distance indicator independent of expansion (Cepheids, Tully-Fisher law, etc.) to demonstrate that we are part of a local supercluster, Laniakea, which contains several galaxy clusters, like Virgo. Cosmic filaments also contain a lot of gas, which is difficult to observe, or is absorbed by distant quasars. Ionized gas recombines to emit the Lyman-alpha line, either because it is excited by star formation, or by fluorescence from quasar light. Observing more and more galaxies (billions of galaxies with the Euclid satellite) will make it possible, thanks to several tools (gravitational lenses, baryonic acoustic waves, RSD redshift-space distortions, etc.) to test the nature of dark energy, by determining its evolution over time, since the Big Bang.
