Étienne Klein guides us on a journey to the heart of mattter. How to explain the fact that substances as different as iron, water, and oxygen are all made up of the same particles? What is radioactivity? What processes led to the universe as we know it today? In answering these questions, Klein helps readers understand the laws at work in the atom's core as well as those governing the movements of galaxies. For the first time, an ensemble view, coherent and accessible to one and all, à tous, fait le point des découvertes sur la matière, des applications technologiques qu'elles rendent possible et des espoirs suscités par les recherches en cours.