I see dark matter
The only reason we can see the Horsehead Nebula is because it is reflecting light from nearby stars. It is non-luminous, ambient, diffuse or specular reflection. If we look directly at the cloud, we can see a few dim points of light from stars that are behind the cloud, directly being blocked by it. Further out on the galactic rim we might not even know the nebula was there, if there aren't enough stars to illuminate it.