A major debate: the Big Bang cosmology - II. The case for the Big Bang

I describe the evidence for the Big Bang, including the various cosmologica l parameters. There is general concordance on their values, although the un certainties remain large enough to justify the ensuing debate. Structure fo rmation is reviewed, and here there is consensus on the hierarchical nature of the sequence of formation. The cosmic microwave background anisotropies provide a glimpse of the initial conditions that seeded structure, and the evolution of clouds of dar matter is an inevitable consequence. There are two outstanding mysteries: the very beginning, where a theory of quantum gr avity is eagerly awaited, and the detailed nature of galaxy formation, wher e observations are well ahead of theory. (C) Academie des sciences/Elsevier , Paris.