This thing we call the luminiferous ether. That is the only substance we are confident of in dynamics. One thing we are sure of, and that is the reality and substantiality of the luminiferous ether. ...Okay, some of this stuff about cracks and jelly seem ridiculous today, but most of it is mostly correct. The discovery of dark energy could be said to be an answer to his question about "whether or not luminiferous ether has gravity".
I move through this “luminiferous ether” as if it were nothing. ... What can this luminiferous ether be? It is something that the planets move through with the greatest ease. It permeates our air; it is nearly in the same condition, so far as our means of judging are concerned, in our air and in the inter-planetary space. The air disturbs it but little; you may reduce air by air-pumps to the hundred thousandth of its density, and you make little effect in the transmission of light through it. The luminiferous ether is an elastic solid, for which the nearest analogy I can give you is this jelly which you see, 5 and the nearest analogy to the waves of light is the motion, which you can imagine, of this elastic jelly, with a ball of wood floating in the middle of it. ...
What we know of the luminiferous ether is that it has the rigidity of a solid and gradually yields. Whether or not it is brittle and cracks we cannot yet tell, but I believe the discoveries in electricity and the motions of comets and the marvellous spurts of light from them, tend to show cracks in the luminiferous ether — show a correspondence between the electric flash and the aurora borealis and cracks in the luminiferous ether. Do not take this as an assertion, it is hardly more than a vague scientific dream: but you may regard the existence of the luminiferous ether as a reality of science; that is, we have an all-pervading medium, an elastic solid, with a great degree of rigidity — an rigidity so prodigious in proportion to its density that the vibrations of light in it have the frequencies I have mentioned, with the wave-lengths I have mentioned. The fundamental question as to whether or not luminiferous ether has gravity has not been answered. We have no knowledge that the luminiferous ether is attracted by gravity; it is sometimes called imponderable because some people vainly imagine that it has no weight; I call it matter with the same kind of rigidity that this elastic jelly has.
Special relativity taught nothing about whether the aether exists. It only taught that any such aether must be Lorentz invariant. As the program conceded at the end, modern quantum field theory does have the concept of a pervasive medium like the aether, as that is how the Higgs field is explained.
The program explained about the Michelson-Morley experiment, and how it failed to detect aether motion. While it was decisive in the discovery and acceptance of special relativity, Einstein seemed to have only vague third-hand knowledge of it.
Yes, it is a historical fact that the MM experiment was crucial for special relativity, but not for Einstein's thinking. Einstein's famous paper was to give an exposition of Lorentz's theory, but it did not bother to explain Lorentz's MM-based reasoning.