This answer is taken from the site of "The Physics Factbook", edited by Glenn Elert, and written by his students....
"The interstellar medium is mainly made of lone hydrogen atoms.
They do not even exist as pairs as they do on earth. I mentioned
before that space is filled with hydrogen atoms. The actual density
of hydrogen as it exists in interstellar space is on the average
of about 1 atom per cubic centimeter. In the extremes, as low
as 0.1 atom per cubic centimeter has been found in the space between
the spiral arms and as high as 1000 atoms per cubic centimeter
are known to exist near the galactic core.
The interstellar medium also contains cosmic dust. These particles
are much bigger than hydrogen atoms. However, there are far fewer
particles of cosmic dust than there are hydrogen atoms in the
same volume of space. It is estimated that cosmic dust is 1000
times less common than hydrogen atoms in the interstellar medium."
May I indulge myself in some speculation for a moment, please? But first, this from an article in "The New World Encyclopedia" to add more to our references:-