Cosmology is the studyof the history of the Universe, it's origin, it's energy content, and the formation of structures that compose it (e.g. galaxies, galaxy clusters). Cosmology is now based on the Big-Bang model of an expanding Universe, after experiencing a hot period. This very simple description of the model ensuing from a number of observational predictions that, once confirmed, validated the comprehension that we now have of our Universe: the flight of the galaxies discovered by Hubble in 1929, the Cosmic Microwave Background radiation at 2,7 K (predicted by Gamow, Alpher and Hermann in 1948 and discovered in 1965 by Penzias and Wilson) and its anisotropies (discovered by Smoot in 1992), and the synthesis of light elements (Helium, Lithium...).
For a few decades, the development of fundamental physics theories and the large observational programs, including the study of anisotropies in the Cosmic Microwave Background, as well as large scale galaxy surveys, allowed the establishment of a rational model relying on a restricted number of parameters, known today with a 10 % precision and heading towards the 1% level soon. In particular, they point out two dark components, dark matter and dark energy, which represent 95% of the energy budget of the Universe. Identifying these components is the challenge which now confronts the scientific community, as a new generation of ground and space-based observational programs are being discussed.