The Interstellar Medium (known as ISM) fills the space between the stars. Space is NOT empty and completely transparent. The ISM is a mix of gases and dust, with gases making up most of it. Hydrogen is the most abundant element in the universe, so the ISM contains more hydrogen than anything else. Approximately 90% of the ISM is hydrogen.
The ISM is NOT evenly distributed in space; it is relatively thick in some areas and quite thin in others. This illustration should make it obvious. This image was made in H-alpha light (656.3 nm) and so shows the hydrogen distribution in the Milky Way's ISM.
When you see something, it means that you can observe it directly, although not necessarily in visible light. Getting an image of something in infra-red, radio, x-rays or other wavelength counts as seeing the object. On the other hand, detecting an object means that you can measure something that tells you that the object is present although you cannot see it directly.
Here's a classic example: you are driving down the road and smell a skunk. You have detected that skunk, because you know it's around somewhere close although you can't see it. When it appears in the road in front of you, then you see it.
Dust is simply very tiny particles of elements that we know as solids. The particles are on the order of 10-7 m. Recall that 10-6 m is known as 1 micron (1000 nm). Green light has a wavelength of about 500 nm, or one half microm. Therefore, the dust particles have a typical size on the order of the wavelength of light, from green to ultraviolet.
There are three ways that dust clouds are seen or detected.
If a cloud of interstellar dust happens to be close enough to a bright star, some of the starlight will be scattered by the dust, making the cloud visible. Look at Figure 11.2. The dust particles will preferentially scatter (bounce) blue light. Here's a way to think about this: imagine running through a crowd. In the first instance, it is a crowd of cats. You are a lot bigger than a cat and can easily jump over them and go straight through. Now imagine a crowd of elephants. These you cannot jump over; you will bounce off them.
Look again at Figure 11.2. If you are looking at a star THROUGH a dust cloud, the dust will scatter some of the blue light, reddening the starlight. The astronomer will see a mismatch between the spectral type of the star (obtained from the absorption lines in the spectrum) and the color. For example, a type A star (white) might appear as type F (slightly yellowish). This is reddening. The amount of reddening can reveal how much dust is there.
Dust in space is cold, but it is NOT at absolute zero; this means that it radiates weakly in the infrared. This is very long wavelength infrared, being somewhere in the 10 to 100 micron range depending on temperature. You can't see this infrared with your eyes, but an infrared-viewing telescope can. By the way, since the Earth's atmosphere is opaque in this region, those telscopes must be in space. A space-borne infrared telescope like IRAS (1980s) or SIRTF (Spitzer Space Telescope) can produce stunning images of dust clouds.
Finally, look at Figure 11.10a. What looks like a hole in space is really a dark dust cloud lying between us and the background stars. The obscuration reveals the dust cloud's presence.
Gas makes up most of the ISM, and 90% of that gas is hydrogen. Another 9 percent is helium. This leaves 1% of other heavier gases (like oxygen, nitrogen, neon, argon, etc.).
The density of the interstellar gas is about 106 atoms per cubic meter, which is 1 atom per cm3. Dust exists at a density of about 1 atom per meter3. This would be a perfect vacuum as far as you are concerned. We can't get anywhere close to this in the laboratory; 109 atoms per cubic meter is considered very good.
There are three ways of seeing or detecting gas clouds in space.
If a gas (hydrogen) cloud is located near a hot young star, ultraviolet radiation from the star will excite the gas atoms and make them fluoresce, or produce emission lines. This makes the cloud directly visible.
On a more subtle note, invisible gas clouds can be detected by using starlight as a probe. An astronomer can study the light from a star and look for very narrow absorption lines, which will be produced by gas between the star and here.
The Horsehead Nebula in Orion is an excellent example of both dark dust and emission nebulae. It is a dust cloud seen silhouetted against a glowing hydrogen cloud. The dust cloud is revealed by counting stars below the horehead and above it. There many more stars above, indicating the presence of obscuring dust below the horsehead in the picture.