2.2. Interstellar Matter: Theoretical Evidences
"There must be more matter than stars......"
-- Jan H. Oort 
In the 1930's Jan H. Oort took another approach to the problem by looking at the statistics of the motions of K giants perpendicular to the plane of the Galaxy, that is, at bulge objects. He used these to estimate the mass of material in the plane. He found that there had to be more material there than could be seen in stars. Oort (1932) estimated that the mass of the non-stellar material (dust and gas) is about 12 × 109 m. If this mass is distributed uniformly, the density of this non-stellar material is ism 6 × 10-24 g cm-3 - this is the mass required to explain the observed motions.
The question then becomes what kind of material distributed with this density with what mass absorption coefficient could give rise to an extinction of about 1mag kpc-1, as observed. So what is required is that the scattering/extinction cross section of the material blocking the light per unit length is on the order of 1 mag kpc-1.