Luminiferous ether was a hypothetical substance once believed to fill space and carry light waves, much like sound waves travel through air. Scientists in the 19th century thought this invisible medium was necessary for the propagation of electromagnetic radiation, including visible light. The concept was largely discredited after the Michelson-Morley experiment in 1887, which failed to detect any evidence of ether. This led to the development of Albert Einstein's theory of relativity, which eliminated the need for ether by explaining that light can travel through a vacuum without a medium.