Cosmology is a very peculiar science since it aims at studying the Universe as a whole. It thus often lies at the frontiers of science, philosophy and metaphysics.
It is the theory of General relativity, established by Einstein at the beginning of the 20th Century, which has allowed to introduce the study of the Universe from a scientific point of view. Thanks to a number of simplifying assumptions (e.g. that on a very large scale, matter and energy can be considered as homogeneously distributed), on derives relatively simple equations, the Friedmann-Lemaître equations, which predict that the Universe is expanding. This expansion was confirmed at the end of the 1920s, with Hubble's discovery that galaxies move apart from each other with a speed proportional tio their distance.
Thus, the Universe would have originated in a very dense and very hot state, which was named the Big Bang. This Big Bang model was dramatically confirmed in 1964 when Penzias and Wilson discovered the cosmic microwave background, which is a relic of a earlier phase when the Universe was about 1000 times hotter than at the present epoch.
However, in order to explain the motions of stars in galaxies, and of galaxies themselves in clusters, it has been necessary to postulate the existence of an unknown form of matter, which does not emit light and is therefore detectable only through its gravitational effect, the famous « dark matter ». This dark matter would be 5 to 6 times more abundant than ordinary matter, made of atoms and molecules.
Moreover, to explain the acceleration of the expansion of the Universe, discovered at the end of the 1990s, cosmologists have postulated the existence of a new form of energy, the so-called « dark energy », which would be the dominant form of energy on a cosmic scale.
The currently favoured cosmological model thus predicts that 95% of the content of the Universe is constituted of « exotic substances » invented to explain cosmological effects not predicted by the basic theory, and still unidentified despite ambitious theoretical and observational research projects.
Rotation curve of a spiral galaxy, explained by introducing a dark matter halo:
Matter-energy content of the Universe: