We have had this discussion before, I even wrote a short paper on it. I think Plasma is just a part of what is happening here and cannot account for all phenomena.
This is a mixture of 2 papers on my blog
http://aaronsreality.blogspot.com/2008/09/plasma-in-intestellar-space.html
and
http://aaronsreality.blogspot.com/2008/10/chain-nature-of-dark-matter.html
I have several issues with Dark Matter having any electrons, ionized He or not. If dark matter turns out to be only protons (ionized H), then I guess I can be ok with that. But I don't think that is what is occurring here.
I need the photon wavefront to move unimpeded from galaxy to galaxy. With the exception of gravitational lensing. but even here we don't lose photons. They are bent or in the case of the Bullet Nebula they prove Reiter's point. They are split (4 times). But we don't lose them to electrons. If the interstellar media is charged particles other than protons (He-, He3, or He4), any electrons would absorb our photons. This is why we see ionized gas when excited by photons. We don't see the original memes here. We see the interaction of the secondary information, the ion releasing a new photon wave.
I agree with almost everything in the first paper
http://www.electric-cosmos.org/electricplasma.htm
but, Pioneer 11 lost velocity while exiting the solar system.
In my model, I see solar systems like bubbles in the ocean. Bubbles can coalesce, bubbles can separate, but there is a difference between inside the bubble and outside the bubble. These are two disparate systems trying to maintain some equilibrium. Its the pressure outside the bubble pressing on the pressure inside the bubble. Solar system are not bubble shapes they are modified torus's.
In the case of the solar system as a bubble, the two systems are (inside) Baryonic in nature and Non-Baryonic in nature (outside). Non-Baryonic particles (photons, free electrons, neutrinos and others) can flow between these two systems. Its the baryons that cannot leave the bubble.
We know that the interstellar media cannot be void.
Now for the missing matter, lets look at our friend Oort and his cloud. I think there is more matter here than previously described. Stellar Nucleosysthesis occurs here. Protons collide here and attract electrons. 3 Alphas can easily form a carbon ion. It starts sliding back towards the sun on the bent dark matter. In stellar lightning speed (a couple of million years) our carbon ion is now a small comet. This process reduces the need for Dark Matter to be heavily massive.
In understanding the nature of memes(information) transmission in baryonic particles it becomes easier to understand the the nature of memes transmission between baryonic particles and dark matter. Memes needs to be transmitted from the baryonic atom to the dark matter particle through magnetism. Dark matter ignores photons because they lack charge. Dark matter reacts to charge, all charges +/- in the same manner. Dark matter is repulsed by charge.
There are 2 known particles that can convert magnetism into memes(information). The electron and the gluon. We can rule out the electron as a particle of dark matter because it has charge. Also if electrons were in intersteller space, photons would never make it through the medium and we would not see other galaxies. The gluon is a more promising particle that could be part of the dark matter chain.
We know that the gluon converts magnetism into color using quantum chromodynamics (QCD). When dark matter is in the presence of magnetism the dark matter is repelled. The interaction between dark matter sub-atomic particles is stronger than the magnetism applied to it. Magnetism is not strong enough to break the normal bond between the gluon and the dark matter particle attached to it. Magnetism bends this bond.
The chain nature of dark matter becomes obvious when gluons are inserted between dark matter particles. These chains form fabrics and the more dense they become the harder is becomes for baryonic particles and photons to move through them. Pioneer 10 and 11 show this in their attempts to exit the solar system.
This above model only requires E/M and baryonic density, sprinkle some thermodynamic and others and it works. As arrogent as it sounds. I think my model is better than Newton, Einstein, and Hawkins put together (MOND). They require attraction through mass. G has never been found. I think CERN is starting to accept this.
Aaron