"Red Shift" is more like the Doppler effect for light. For those of you who don't remember the Doppler effect from high school physics, it's the reason that a car horn seems to drop in pitch as it drives by. The red shift doesn't have to do with the fact that the light is moving through matter (because most of space is a vaccuum anyway.) The light is shifted toward red because as the star moves away, the wavelengths appear lengthened to us; likewise, as a star moves closer, the light wavelengths are smashed closer together, hence the "blue shift."