L'Univers à grande échelle Les grandes structures et leur formation

A few years ago, the elaboration of catalogues of galaxies based on decades of observations seemed to give the clue of the actual distribution of matter in the Universe. Moreover, the study of the growth by gravitational instability of initial inhomogeneities spread out on a globally homogeneous cosmological background gave some light on the formation of galaxies. However, technological developments allowing the obtention of large samples of recessional velocities of galaxies leading via the Hubble law to the 3rd dimension have modified the preceding vision. Galaxies seem now to cluster in large aggregates (clusters and superclusters) organized in an interconnected network with large voids, empty of bright galaxies. Besides, if galaxy formation occurs only in specially selected regions, this knowledge of the large scale structure of the Universe does not reflect the true distribution of matter ! One loophole comes probably from elementary particle physics. The effort to unify fundamental interactions appears to have profound cosmological implications. Perhaps, the matter of our world (baryons) is only a negligible part of the (dark) matter constituted of axions, or neutrinos, photinos... governing the dynamics of the whole Universe and these « cold » or « hot » « inos » thus play a main role in the formation of the observed structures. In this domain, things evolve very rapidly, and this paper tends to give a snapshot of the strange situation to which astrophysicians are now confronted. Stellar studies have precedingly shown the close link between stellar and atomic scales. If confirmed, the recent developments of Cosmology and Particle Physics seem to establish the ultimate connection between the Cosmos and the microworld