Good, now we are getting somewhere. Now, by how much will the speed of light vary as it goes through, say, the Orion nebula? I will accept a ball-park percentage.<quoted text>
If it's in an open vacuum, it doesn't vary.
But if it hits a nebula or speck of space dust, it varies.
Would it be as much as the speed varies as light goes through, say, air?
Recall that air has a refractive index of 1.000293, so the decrease in the speed of light through air is about .03% from that in a vacuum. For helium (at standard pressure and temperature) the reduction is about .004%.
Just so you have a picture: