This book describes some of the frontier problems of cosmology: our almost total ignorance of what the Universe is made up of, the mystery of its origin and its end. The book starts with a description of the historical events that led to the construction of the Big Bang model together with the stages that transformed the Universe from a very hot place to a very cold one, full with the structures that we observe today. These structures (stars, galaxies, etc.) constitute only 5% of the contents of the Universe. Concerning the remaining 95%, dubbed dark matter and dark energy, we know very...