Dark Energy and Dark Matter

About ninety years ago the general belief was that the universe was fairly understood. The general theory of relativity and the expansion of the universe successfully described the dynamics of stars and galaxies and even the large-scale structure of the universe. Questions, however, arose in the thirties of the last century, when Fritz Zwicky raised the question how the galaxies managed to remain stable, given the large rotational velocities of stars at the edges of the Milky Way. With the then known amount of gravitational matter stars could easily escape from the galaxy and the whole galaxy would disintegrate.