As far as I know, Dark Matter is not at all related to ether. Ether was supposed to be the medium through which light traveled (Light is a wave, so it’s gotta be a wave in something, they reasoned). It turns out that no such medium exists.
Dark Matter is a theorized amount of matter in the universe that does not emit light. Based on observations of galactic motion, there must be some mass that is not in stars. Quite a lot, in fact.
This mass may be in any of the following forms (or in none of them. These are just the theories I can recall right now)
WIMPS (Weaking Interacting Massive Particles): The idea is that there’s some elementary particles out there (on the scale of bosons, so, made of quarks) that don’t interact electromagnetically very much, but are very massive.
Brown Dwarfs: There’s lots of skars out there that give off very little light, or none at all.
Gas Clouds: There’s lots of gas and dust out there that’s not dense enough to form stars yet, but still has lots of mass in it.
Dark Matter and ether are different concepts. The ether is the supposed medium through which light travels. The speed of light is c with respect to the ether. Relativity denies absolute velocity, and so does not allow for the existence of ether. The speed of light is c with respect to any inertial reference frame.
The rotation of the galaxies is not what would be expected if only the mass of the stars is considered. A halo of Dark Matter/Energy could account for the discrepancy.
The luminiferous ether was theorized to exist everywhere and serve as the medium through which light propagated. In contrast, dark matter forms clumps. Here’s a map of dark matter distribution in a cluster of galaxies, and here’s a map of its distribution between galactic clusters. The distribution of the stuff within single galaxies is still apparently the subject of debate, although almost by definition, it needs to be situated so as to eliminate the anomalous orbital velocities of stars within any given galaxy.
If dark matter bore any relation to the luminiferous ether, as a medium for the propagation of photons, the universe would look considerably different than it does. To see why, ask yourself what the photons would do when they came up against an area containing no dark matter at all. How would we see past such an area ?
According to Scientific American, the most likely constituent of dark matter is an exotic particle called the “neutralino”, the lightest of the stable super-partners from supersymmetry.
“Dark energy”, an awful term, is simply one theory which attempts to explain observations that seem to show that the rate at which the universe is expanding is speeding up over time. Whether this increased expansion is due to some hitherto unknown force, oddities in the behavior of gravity, or just the universe relaxing as it gets older, and thus allowing more space to leak in from neighboring branes, is still unknown.
In case people get the idea that dark matter is a theoretical concept, remember: we are dark matter! Dark matter is the discrepancy in the mass of the galaxy deduced from orbital motion and the total mass of stars deduced from the amount of light emitted. Planets don’t emit light, so planets (and the gunk on their surfaces, like us) are dark matter.
But there probably aren’t enough planets to account for all the mass, so there are other forms of dark matter.