Either of these has a much smaller density than the best vacuum that can be created in the laboratory. We don't have much direct knowledge of what they are made of since the particles very tiny. Like individual grains of tobacco smoke rather than sand grains. A large fraction of this dust contains some of the elements. This is called extinction.
Thick clouds containing dust can make background stars invisible.
This is called a dark nebula . If the dust cloud is not too thick the light will only be partly dimmed. This is a limitation on the use of Wien 's Law in determining stellar temperatures. The same effect causes the Sun to appear red at Sunset. The effect is stronger at sunset because the light passes through more of the Earth's atmosphere when the sun is near the horizon.