Luminiferous aether
Luminiferous aether' is something 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 traveled through something. Sound waves traveled faster through steel than through air. So people imagined that light had to travel through something too. But at the same time this substance would have to be entirely yielding so that it would not slow down the movements of planets and cause them to eventually fall into their suns.