According to ancient and
medieval science,
aether (
aither), also spelled
æther or
ether, also called
quintessence, is the material that fills the region of the
universe above the
terrestrial sphere. The concept of aether was used in several theories to explain several natural phenomena, such as the traveling of light and gravity. In the late 19th century, physicists postulated that aether permeated all throughout space, providing a medium through which light could travel in a
vacuum, but evidence for the presence of such a medium was not found in the
Michelson–Morley experiment.