The recent discovery that the expansion of the universe is accelerating due to an unknown “dark energy” highlights the limited understanding of the major constituents of the universe. This surprising result, along with the finding that most matter is invisible, sets the stage for research in cosmology at the start of the 21st century: the study of the distribution of “dark matter” and the properties of the dark energy are among the most important problems in astronomy. It is a data-driven endeavour since we lack a clear theoretical framework. The objective of this proposal is to advance observational constraints to a level where physical mechanisms that could underlie the dark energy can be distinguished. The method of choice for this proposal is a relatively new technique called “weak gravitational lensing”, which allows one to “map” the distribution of dark matter and how it evolves with time. It also provides a unique way to study the mass distribution in massive clusters of galaxies. Understanding the formation and physical properties of these rare systems is mandatory, if their number density as a function of mass and redshift is to be used as a cosmological tool. The aim of this proposal is to fully exploit the large amounts of world-class data the applicant has access to. The resulting measurements of the dark energy properties will be among the most accurate ones available, and this work will be an important step forward to reach the ultimate goal of percent level accuracy. In addition, a number of ancillary projects, in particular the study of dark matter halos around galaxies, will provide unique constraints on the properties of dark matter.