In 1704, Sir Isaac Newton, both a scientist and an alchemist, published Opticks, a theory of light as a particle. This was a huge step forward in understanding light, reflection, etc, but he had a few holes in his theory. In attempting to explain refraction, he had trouble, and decided there must be an "Aetherial Medium" through which light traveled.
Both Fresnel and Newton concluded there must be this invisible Aether all around us, despite having otherwise contradictory notions of what light was. For the next 70 years, all of science accepted the notion of Lumineferous Aether, the invisible medium in which light propagated. It was all around us, we moved through it and dragged it behind us, but we could neither see nor feel it. It's "existence" made certain concepts easier to understand, but it also had problems.
Starting in 1887, a series of experiments began to show flaws in the aetheric theory. Conflicts arose between James Clerk Maxwell's theory of Electromagnetism and Newton's dynamics. In 1904, Hendrik Lorentz proposed that the Aether did not move at all, and mass didn't drag it. A year later, Albert Einstein published his theory of Special Relativity, which solved a number of issues concerning light and motion, using Lorentz's calculations but without needing any aether at all. This stirred up some debate, but Relativity was quickly adopted by the scientific community. Lumineferous Aether has been largely ignored by mainstream science, as soon as experiments began to show that Relativity was sound and correct. A few tests intended to prove the existence of the aether have been inconclusive or even provided some evidence suggesting that there is no aether. It's not so much been disproved as simply shown to be redundant and unnecessary.
Occam's Razor says we should focus on the least complicated theory that explains a given phenomenon, and the main theories explaining light and the universe don't require an aether. Light, we now understand, is a particle with wavelike properties, and that works plenty fine for all practical purposes without there needing to be some invisible aether through which it vibrates.
Game and Story Use
- Science has concluded there doesn't need to be a Lumineferous Aether. Then again science has largely ruled out FTL, ghosts, and Magic, any of which could (in a game or story) somehow be related to "other purposes" or alternate "vibrations" of the aether that we've just overlooked.
- An Alternate History that never discovered Relativity would probably lack nuclear power, and tail behind us in technology in general. It's "primitive" aether-driven science might combine with the Victorian-era interest in the occult to include technomancy, machines that talk to the dead and other fabulous steampunk contraptions.
- String theory proposes that light is a vibration in the fifth dimension (ie: the fourth spatial dimension).  Does this mean the Aether is 5-dimensional?
- If so, the fourth dimensional lifeform might have a few more tricks up his sleeve than that page already suggests. It might be able to bend, create, or capture light, change the colors of things, create reflections and holograms, etc. Of course, that's small potatoes compared to what a fourth dimensional lifeform can already do without the aether, but it's still kinda cool.
- Because there's still a little bit of debate about whether or not the Lumineferous Aether exists, it's a way you can have a scientist character dabble in alternative science or superscience without characterizing them as a Mad Scientist or Agent Mulder.
- In a Victorian or Pulp Era Science Fiction setting, reference to the Aether, particularly in reference to radio waves, can add a bit of period flavor.
- Some Golden Age science fiction writers, like Isaac Asimov, would use "sub-ether" as a term for FTL Radio