Perhaps it should be noted that the most common form of matter in the universe is plasma. Two interesting properties of plasma are 1) Light is red shifted when travelling through it. 2)Light is refracted when passing through plasma.

The fact that light is refracted throws a spanner into the works for Dark Matter as plasma likely contributes to gravitational lensing and estimates for the amount of mass present. Plasma may be there but produce no synchrohtron radiation.