Ordinary interstellar space holds about one atom per cubic centimetre. The best vacuum that can be made in a laboratory is around 10^{10} atoms per cubic centimetre, so space is ten billion times emptier than the emptiest thing we can build.
What makes it matter is the volume. The distances are so large that the total adds up to something like ten percent of the mass of all the stars in the galaxy put together.
The temperature runs over a wide range. Cold clouds sit at 10 to 100 K. Gas near hot stars is ionised and runs at about 10^{4} K. Gas swept up by supernovae reaches 10^{6} K and fills large bubbles between everything else.
About one percent of the mass is not gas at all but solid grains of carbon and silicate, roughly the size of the wavelength of light. That one percent is responsible for most of what we cannot see.
Grains that are about as big as the wavelength of light scatter and absorb it, and they do it more strongly at short wavelengths than at long ones. So light coming through a dusty region is not only dimmed but reddened.
Dimming alone would be a nuisance. A star behind dust looks fainter than it is, so the distance modulus overestimates its distance, and any measurement built on brightness goes wrong.
Reddening is what saves the situation. Dimming and reddening go together in a fixed ratio, so measuring the color index tells us how much reddening there is, and from that how much dimming to correct for.
This is why the color index of the previous article does double duty. Compare the color a star should have from its spectral type with the color it actually shows, and the difference is a measure of the dust in the way.
A cloud has no light of its own. How it appears depends entirely on what happens to be near it, and there are three cases.
If a hot star sits inside it, the ultraviolet from that star strips the electrons off the hydrogen. When the electrons find their way back down, the cloud emits light of its own, at the particular wavelengths hydrogen produces. This is an emission nebula, and the red of hydrogen is what gives these their color.
If there is a star nearby but not hot enough to ionise anything, the cloud simply scatters the light that reaches it. Short wavelengths scatter best, so the cloud looks blue. This is a reflection nebula, and it is the same effect that makes the sky blue.
If there is no star near it at all, the cloud only blocks what is behind. It shows as a hole in the star field. These dark nebulae look like gaps in the galaxy, and for a long time that is what they were taken to be.
Cold neutral hydrogen emits almost no visible light. It is the most common substance in the galaxy and it was, for a long time, close to invisible.
The way to see it comes from a small detail. The proton and the electron each behave as tiny magnets, and the atom holds slightly more energy when the two point the same way than when they point opposite. Dropping from one to the other gives up that small difference as a photon, and the wavelength of that photon is 21 cm.
For any single atom the flip happens about once in ten million years, which is hopeless. But the number of atoms is so large that the galaxy glows steadily at that wavelength.
The wavelength is far longer than the dust grains, so the dust does not touch it. Radio at 21 cm passes through the whole galaxy, including the parts of the disc that visible light cannot cross, and it is what the map of our own galaxy was drawn from.
The material is not used once and finished with. Clouds collapse into stars, stars build heavier elements inside, stars end and hand the material back, and what comes back becomes the next generation of clouds.
The return happens in two ways. Light stars shed their outer layers gently as planetary nebulae. Heavy stars blow theirs off in supernovae, and those also compress the gas nearby, which sets off the next round of collapse.
The cycle is not a loop that comes back to the same place. Each turn the returned material carries more of the heavy elements that were built in the last generation, so the next clouds start out richer than the ones before.
The sun formed from material that had already been through this many times. Everything in the earth that is not hydrogen or helium was made inside a star that ended before the sun began, and the same is true of the calcium in our bones and the iron in our blood.