There was a "Sky at Night" programme on BBC TV this week, it will be on the iPlayer.

Basically they reckoned intersteller gas made up at least 70% of Dark Matter. They demonstrated the problem of seeing it by analogy:

A fine spray of water particles is hard to see at a distance, but shine a light through it and you see by light scattering.

The cosmic light source is a pulsar, the gas molecules get illuminated. And with enough pulsars around and probably an extrapolation get you to 70%. Gasses are not necessarily evenly distributed.