Everything Science Forum

The space in between the stars is not completely empty, but is instead filled with a tenuous mix of gas and dust most generally referred to as the interstellar medium (ISM). The ISM plays a crucial role in astrophysics precisely because of its intermediate role between stellar and galactic scales.   For many astronomers, the ISM is nothing more than an annoyance because it obscures our view of distant astronomical sources, but the ISM is well worth studying in its own right.

Combined quote from -

Interstellar Medium (http://origins.colorado.edu/~woodb/resint2/resint2.html)  - this article refers to LISM, local interstellar medium

http://en.wikipedia.org/wiki/Interstellar_medium (http://en.wikipedia.org/wiki/Interstellar_medium)

Features prominent in the study of the interstellar medium include molecular clouds, interstellar clouds, supernova remnants, planetary nebulae, and similar diffuse structures.

The space between the stars is not empty, but full of gas and dust. In fact, the net mass of the interstellar gas in the Milky Way galaxy is about 10 - 20% of the net mass in stars. Like the matter in the Sun and most stars, the interstellar gas is composed mostly of hydrogen (about 70% by mass), helium (about 29%), and just a trace (about 1%) of heavier elements such as carbon, nitrogen, oxygen, silicon, iron, etc. -- the stuff the Earth and your bodies are made of.

This gas has a wide range of physical conditions. Half of interstellar space is filled with diffuse interstellar gas, having atomic density ranging from about 1 - 100 atoms/cm³. Most of the diffuse gas has temperature of about 100 K (= -280?F).

http://cosmos.colorado.edu/stem/courses/common/documents/chapter9/l9S1.htm (http://cosmos.colorado.edu/stem/courses/common/documents/chapter9/l9S1.htm)

Intergalactic medium - somewhat related to Interstellar medium

http://en.wikipedia.org/wiki/Intergalactic_medium

One of the other wonders of Scorpius is just a few degrees to the east of Antares. But you can't see it -- at least not yet. It's a black cloud that may someday give birth to a new star.

The cloud is Barnard 68. It's named for American astronomer Edward Emerson Barnard, who cataloged it in 1919. It's about 300 to 400 light-years from Earth -- a bit closer than Antares.

The cloud consists mostly of hydrogen and helium, two gases that are, strangely enough, transparent. But about one percent of the cloud is made of dust, which blocks the light from the stars beyond it.

Barnard 68 is five light-months across. In other words, if the dust didn't absorb it, light would take five months to speed from one side of the cloud to the other. If you centered the cloud on the solar system, it would extend 12,000 times farther from the Sun than Earth's orbit.

Barnard 68 is frigid: 440 degrees below zero Fahrenheit -- colder than Pluto. At that temperature, its gas exerts little outward pressure. As a result, the inward pull of gravity may make the cloud collapse.

from http://stardate.org/radio/program.php?f=detail&id=2008-07-13 (http://stardate.org/radio/program.php?f=detail&id=2008-07-13)

Here is a list of dark nubulae!
http://en.wikipedia.org/wiki/List_of_dark_nebulae (http://en.wikipedia.org/wiki/List_of_dark_nebulae)
 
Some the more famous or well-known interstellar clouds:
 
Barnard 33 - Horsehead Nebula (http://en.wikipedia.org/wiki/Horsehead_Nebula)
Barnard 67 (http://en.wikipedia.org/wiki/Barnard_67) - part of Pipe Nebula (http://en.wikipedia.org/wiki/Pipe_Nebula) (Barnard 59, 65?67, and 78)
Barnard 72 - Snake Nebula (http://en.wikipedia.org/wiki/Snake_Nebula)
Barnard 85 (http://en.wikipedia.org/w/index.php?title=Barnard_85&action=edit&redlink=1), dark part of the Trifid Nebula (http://en.wikipedia.org/wiki/Trifid_Nebula)
Barnard 88 (http://en.wikipedia.org/w/index.php?title=Barnard_88&action=edit&redlink=1), dark part of the Lagoon Nebula (http://en.wikipedia.org/wiki/Lagoon_Nebula)
 
These are certainly interesting and challenging objects to understand.