Question: Hi, what is space made out of?

This question used to be a real headache for cosmologists. The problem was light. Light was, for centuries, thought to be a wave and waves have to travel through something, so space must be filled with something that was completely invisible but capable of carrying the light. So the idea of the Luminiferous Aether was born. The problem was the more research was done the weirder and weirder the properties of the Aether became. It seemed to be everywhere, but couldn’t be seen or captured. It was everywhere equally, so didn’t obey gravity and din’t have mass. To carry light with the right frequencies it had to be many times stiffer than steel, but it also had to offer no friction to the passage of the planets through it, or the planets would slow and spiral into the sun. And it could have no viscosity – it didn’t seem to pile up in front of the planets or break down into vorticies behind them because the light carried straight on as before. Eventually it became too much, the properties of the aether became too bizarre to be believable. The idea was clearly nonsense, but it was only in the early to mid 20th century that a better idea came along – that light was both a wave and a particle. Particles are things unto themselves, they don’t need a medium to travel through. So the luminiferous aether, one of the great flawed ideas of science, passed into history.