Throughout the history of astronomy, various thinkers, philosophers and scientists have postulated the existence of entities which, not being visible or detectable in their time, or perhaps ever, were however necessary to maintain the stability of the cosmos, as they saw and understood it.

Since Swiss astronomers Michel Mayor and Didier Queloz announced the discovery of the first extrasolar planet or exoplanet, in 1995, different research groups worldwide

In ancient Greece, Empedocles stated that everything was made up of only four elements: water, fire, earth and air. It was undoubtedly the first attempt to describe the content of our universe. Over a hundred years later, Aristotle added a fifth element: ether or quintessence, which formed the stars, while the other four were the constituents of terrestrial substances.