What is it and why was it proposed?
Light is a wave, and as such, it requires a medium through which to propagate. In other words, nothing to vibrate means no vibrations. So they sought to call the aether, an ancient idea predating the ancient Greeks, by some other name than just simply the aether to illustrate its importance to light.

Rejected:
A physicist and a chemist by the names of Michelson and Morley couldn't find a change in the velocity of light as the earth moved around the sun. The aether was supposed to be stationary, and if it were, they postulated that the earth must move through the aether at different speeds. This would imply that the speed of light relative to the earth would change during the course of the year as the earth rotated around the sun. Long story short, they didn't find a difference in the speeds of light taken during different seasons, Einstein plagiarized the results of Lorentz, and now very few people believe in the existence of the aether.

I personally find it asinine to not believe there is an aether, but you didn't ask about my opinions.

I totally agree: said aether does (and must) exist...if not solely for the case of dark matter.

I am quite glad to hear someone agree with the idea that the aether is responsible for the dark matter in the universe. Lord Kelvin calculated the mass of a volume of aether one centimeter by one centimeter by one light-year to be 4.4 grams, and astrophysicists estimate that the intergalactic medium has a density equivalent to one hydrogen atom per cubic meter.

I just can't get over how cool it is to hear someone else saying what I have long believed to be true. Others have agreed that there is an aether, but to hear someone else bring dark matter into the discussion fills me with hope for the future.

I do know that the ether exist I found the ether wind and can prove it and I do know how gravity works.

I not only agree, I have a new theory on the composition and functionality of the Luminiferous Aether that has never been proposed before, but I'm having a difficult time trying to have it reviewed by a peer group because it cost so much I can't afford it.

I suggest that geodesic paths of gravitational motion assumed to be caused by a curvature of space; paths of light observed as being bent in a gravitational field; the red shift in an observed light spectrum as it traverses outward in the gravitational field of its source; the red shift in distant galaxies light spectrum assumed to be primarily due to the Doppler Effect caused by receding galaxies in a rapidly expanding universe; the quantum mechanics of gravity; dark energy; dark matter; and the relativity of time and energy; are all unified in the substance of an accurately defined universal “luminiferous aether” (ether) as proposed by British physicist James Clerk Maxwell.

With the dismissal of Maxwell’s “luminiferous aether” as a result of the famous MMX and then with the resounding success of Einstein’s Special Theory of Relativity, both of which are held to disprove the need of and even the existence of such a substance to facilitate the propagation of light, any further scientific investigatory research to discover if such a substance exists has been all but abandoned. At the time of the introduction of the ether theory, and even by the time of Einstein’s special and general theories of relativity, many of the now proven principles of EM radiation were not known. Based upon an in-depth analysis using all proven data of now known properties of radiated EM energy, I propose the composition and functionality of the elusive ether is now defined in a copyrighted paper I have registered with the Library of Congress and is consistent with all experiments testing the phenomena of special relativity, general relativity, and relativistic quantum mechanics.

As defined in this paper the universal ether is suggested as what constitutes the composition of the “dark energy" glue that ties all four interaction forces in Nature together, bonding all atomic components in atomic structures together in a quantum entropy pattern of motion as required to sustain their interdependent coexistence, facilitates the propagation of EM radiation through space, and accounts for the continual creation of new matter as suggested in the Bondi/Gold/Hoyle steady state theory to maintain an unending existence of the universe. I suggest the new matter thus created constitutes the substance of “dark matter.”