Aether

Aether (also spelled ether) is a concept used in ancient and medieval science as a substance. The aether was believed to be the substance which filled the region of the universe above the terrestrial sphere. Aristotle included it as a fifth element distinct from the other four, Air, Earth, Fire, and Water. Aether was also called Quintessence (from quinta essentia, “fifth element”). Quintessence was also supposed to be a definition of pure energy. Its force is imagined to be like a lightning. This element also has the power of life. Its Platonic solid was the Dodecahedron.

Modern understanding of electromagnetism, including Einstein’s particle theory of light and various scientific experiments of general relativity, has removed the need for a substance like aether to fill the otherwise empty parts of the universe. Newton’s and Maxwell’s aether model (the latter being a “classic static aether”) were both developed from this classical element. However, the null result of the Michelson-Morley experiment led (from 1887 onwards) to the decline of an aether model’s wide acceptance. Albert Einstein, in an interpretation he offered for his theory of special relativity, dismissed it, as per Occam’s razor; and, though he later reinstated a logical need for an aether in a commentary on his theory of general relativity, modern astrophysical theories do not include this classical element. One might suppose ‘dark matter’ has supplanted “aether.”

Taken From Wikipedia