In this course, we will highlight the synergy between studies of the largest and smallest observable length scales. As we will see throughout the lectures, the tools we developed by investigating Nature at the smallest observable length scales allowed us to come to a consistent picture of the Universe. According to such a picture, the fundamental constituents are baryons, dark matter, and dark energy, supplemented by initial conditions determined by an early epoch of inflation. However, many questions are still unanswered. The aim of this course is to make students aware of such issues, review the most common solutions and potentially discuss novel ideas suited for a master thesis project.