Eckard,
Interesting...
RE: "Michelson's 1881/1887 experiment... and at least the following alternatives were considered":
1- a totally or just partially dragged medium fully or nearly, respectively, stationary to earth was and is not convincing for various reasons.
2- length contraction was suggested by FitzGerald and Lorentz as to rescue the light-carrying medium and adopted by Poincaré as well as Einstein.
3- a Newtonian ballistic, so called emitter theory was preferred by Ritz and later on defended by means of reemission arguments.
4-And your proposal that electromagnetic waves don't at all need a medium in which they propagate but their speed in empty space relates to distances and a common time.
Alternatives 3 has been somehow discredited. Your proposal 4 suggests that electromagnetic waves have to know the distance they will travel in advance in order that they will arrive at a common time, not too early and not too late. Just on time! This is much unlike other waves. Alternative 2 is the currently accepted explanation and has come with so much theoretical headache and philosophical paradoxes. Those suffering from the headache are currently looking for analgesic because it refuses to go. And to make things worse the foundation is becoming shaky as Einstein's writings have become accesible to all on the internet with the expiry of their copyright under the 100 years limit and many inconsistencies covered up are coming to light. I have on this site pointed out the falsity of the absolute value of light speed being "exactly" 299792458m/s. Now for alternative 1 that would have saved us all the headache and preserved headache-free Galilean relativity, no one can really be blamed. There was no identifiable and consistent candidate for an earth-bound medium in 1887 and 1905. But now there is one that its abundance is bewildering, even our Sun bows down to it in its galactic motion. It does not need to be dragged like ether since it interacts gravitationally and so can be earth-bound. This has now become respectable with some published papers on this theme. Just Google "earth bound dark matter" to see a few.
To your question, whether light propagates in dark matter, this is common knowledge (if dark matter exists), if not given its abundance most stars will be invisible to us in the Milky way. So light can pass through dark matter and if it does so as other type of waves, then dark matter is like any other light carrying matter medium. What Michelson's null result refutes is that on earth surface light does not move relative to earth, the why is the Alternatives 1-4. The deciding point is what of light not on the surface of the earth, does it move relative to earth? The answer is a resounding Yes, it does. So Alternative 2 is in trouble. Examples are seen in the anisotropy of the cosmic background radiation showing that optical phenomena can be used to determine the earth's motion at 380m/s, contrary to Einstein's postulate (read the 1905 paper). Nearer home, earth motion is also observable in laser ranging type of experiments not exclusive to earth surface, but this motion which Alternative 2 had hitherto said is unobservable is edited out of recordings by referring the ranging results to what is technically called the solar system barycentre. Why use a barycentric dynamic time (TDB) when you are an earth-based observer, with a clock of your own?
Response getting long so let me stop here meanwhile..
Regards,
Akinbo
