Jim,
You asked; "Wouldn't a Milky Way "Dark Matter halo"...that was actually composed of baryonic WHIM or whatever, necessarily emit EM radiation? At the specified densities, why would such massive halos, comprising up to 90% of total galactic mass, not be detected?
I'm sorry if you felt I hadn't answered that, but I wrote; it was because: "Detectable' does not imply detected" so our instruments are simply not yet quite sensitive enough for easy direct detection, but; "Improving instruments detect more and more."
I also repeated my previous point that plasma ions ARE baryonic matter, and' "Plasma n=1, so ions absorb and re-emit EM radiation, but don't change it unless moving, so don't otherwise give themselves away."
I'd quite forgotten how prickly and aggressive you could be, but I'm struggling to see how I can more directly answer your question. I get the impression you just react badly to anything inconsistent with what you believe. You thus entirely misinterpret my manner as 'demeaning'. I'm sorry if it seems it is, and if I went on to also give better background, but I'm only trying to help. In really direct terms;
NO, baryonic matter does NOT normally "emit" EM radiation, it absorbs and "RE-emits" it (like all refraction and reflection), and NO, if it is plasma we would NOT then 'detect' any change in the radiation (i.e. we would not 'detect the presence of' the plasma), and certainly not in the optical range.
Having said that, where there is pair production (ions) there is then also some (bound) molecular gas (baryonic), which IS increasingly 'detectable'. I confirmed there are a number of other ways both CAN be directly detected including;
Kinetics (the particles move so affect lambda).
Flaring (I've found a free link; APJLett732 2011).
Direct at different wavelengths; i.e. sub.mm; See my Fig 1 here; Centaurus A
I try not to seek 'support' for my postulations but genuine falsification. More consistent re-interpretation of findings is the heart scientific progress, and is indeed what you are doing, but please do help by identifying any specific point where you think I haven't been objective or consistent..
Best wishes
Peter