Visible Matter

How do we work out the amount of matter inferred by the light we see in the universe? How do we work out the amount of matter from the light?

The little I remember about spectroscopy was that you could find the amount of stuff by the intensity of absorption bands-but that was small samples in equilibrium. Looking in any direction into space gasses are at different temperatures so we`ve got emission bands superimposed on absorption bands and vice-versa. So surely using spectroscopy we`re only looking at the difference between emission and absorption and there isn`t a way of determining the amounts of material spectroscopically. So is there another way of finding the amount of matter using light, or is it safe to ignore superimposition?

This is such a basic question that the 400 page book I bought to answer this question didn`t mention it- Or will somebody clear up my confusion?