No, they can detect plasma, and there is plenty of it all over the universe. The plasma universe theorists don’t think that the plasma IS the dark matter. What they think is that the dark matter isn’t necessary at all, once you figure in the electromagnetic effects into the equations.
You see, the dark matter is only necessary because a galaxy’s motion does not behave as it should if the motion is caused only by gravity. If gravity was causing it, then the outer parts of the galaxy would revolve more slowly than the inner parts, but instead we see that the outer parts of the galaxy keep the same angular velocity as the inner parts. So, the standard model theorists try to resolve this by proposing there is a whole lot more mass in the universe that we just can’t detect, and also by putting black holes in the center of galaxies to increase the mass as well.
The plasma theorists say that the galactic motion is entirely explainable if you understand the galaxies are made up mostly of plasma, and the plasma is moving mostly due to electromagnetic forces, and not due to gravity alone.
I suspect they've all overlooked something which brings me to another theory that's just a plausible; maybe moreso, than any of the theirs: God designed it, built it and maintains it; something for Him that takes little more than an afterthought. I also suspect God has a sense of humor and gets a chuckle out of watching the Scientific Community™ and their efforts to explain His work by following the trail of bread crumbs He left behind. Just a theory mind you...