Luminiferous aether is a substance once believed to fill the Universe and explain how the transmission of waves of light can happen. People believed that light was a kind of wave. Waves travel over the surface of water in a pond, through the air in a room, etc. All waves that people had studied before had travelled through a medium. The speed of sound through steel is much higher than through air, so people imagined that light must travel through a similar medium too. Since light travels faster than anything else, light should travel through something more rigid (like steel) faster than anything else. But at the same time this substance must have a very low viscosity so that it would not slow down the movements of planets and cause them to eventually fall into their suns. The people coming before Albert Einstein believed this substance to exist, and they called it the "Luminiferous aether." 