Aether

Light is a wave, and the luminiferous aether (or ether) is defined to be whatever medium transmits the light waves.

The Michelson-Morley experiment in 1887 failed to detect the aether. H. Poincare conjectured that the aether may be unobservable, and invented special relativity based on the notion that the aether may be superfluous. A. Einstein published conflicting opinions on whether the aether exists.

Today we have no firm answer on whether the aether exists or not. Quantum electrodynamics (QED) teaches that when light goes through a vacuum, it is really going through a sea of virtual particles. Cosmologists say that the vacuum is really filled with dark energy, and it is unknown whether that has any relation to the aether concept. The pre-relativity aether theory has been superseded, but the precise nature of the aether is an open question.