Luminiferous aether was a theoretical substance once believed to fill empty space and serve as the medium through which light waves traveled. Scientists in the 19th century proposed that this invisible aether was necessary for the propagation of electromagnetic waves, similar to how sound waves travel through air. The concept of luminiferous aether was largely abandoned after the Michelson-Morley experiment in 1887 failed to detect its presence. This experiment contributed to the development of Albert Einstein's theory of relativity, which eliminated the need for a medium like aether to explain the behavior of light and other electromagnetic radiation.