|MadSci Network: Astronomy|
By black areas in space I understand that you mean the dark spaces that appear among stars in photographs of space. This was a question that puzzled astronomers for a long time and was not answered satisfactorily until 70 years ago. We now know that ALL space in the Galaxy has some amount of dust and gas. The dust consists of microscopic grains made mostly of silicate, carbon or iron. The gas is mostly hydrogen, with some ten percent of helium and other elements like oxygen, nitrogen and carbon in much smaller proportion. This mixture of gas and dust is called interstellar matter (meaning matter among stars).
The interstellar matter that is near a star and that is dense enough appears in telescopes as a bright patch or cloud, called a nebula (this is a Latin word, nebula means cloud, nebulaE means cloudS). If the star is very hot (a blue star), the gas glows and the nebula becomes very bright. (This is because the gas reflects light from the star toward us, and because the gas emits some light on its own.)
The interstellar matter that is not illuminated by a star does not reflect or produce visible light, but can block the light that comes from stars behind it, and therefore looks like a dark patch in space. These dark nebulae can also emit radio waves and infrared light, which can be detected with special telescopes and detectors on Earth and in artificial satellites in orbit.
The interstellar matter is not evenly distributed in space. It can form clouds of can be very sparsely distributed. The densest clouds of interstellar matter can have up to 10 million particles per cubic inch while its least dense parts can have a few particles in a cubic inch. You can see many examples of nebula, both dark and bright in http://antwrp.gsfc.nasa. gov/apod/lib/aptree.html
Vladimir Escalante Ramirez
[Moderator's Note: Ten million particles per cubic inch sounds like a lot - but actually space is very, very empty! This is because our atmosphere on earth has about 100,000,000,000,000,000 particles - that's one hundred million billion particles.]
Try the links in the MadSci Library for more information on Astronomy.