Luminiferous æther was a theoretical substance once believed to fill empty space and carry light waves. Scientists in the 19th century thought it was necessary for the propagation of electromagnetic waves, similar to how sound waves travel through air. This concept helped explain how light could travel through a vacuum. However, experiments, such as the famous Michelson-Morley experiment, failed to detect the æther, leading to its decline in scientific acceptance. The development of Albert Einstein's theory of relativity in the early 20th century provided a new understanding of light and space, rendering the concept of luminiferous æther unnecessary.