Most common material in the universe
The Big Bang started with about 3/4 hydrogen atoms and 1/4 helium atoms.  Add a pinch of lithium and a whisper of boron.  Supernovae since then have released a lot of helium.  Heavier elements are still rare.

"The most common [molecular] material in the universe" must survive photolysis, radiolysis, and reaction.  Water is poor candidate.

http://www2.astro.psu.edu/users/alex/astro497_7.pdf 
What is out there.  CO is the marker. 
http://physics.nist.gov/cgi-bin/micro/table5/start.pl 
Molecules in space, isotopically and by 
http://science.gsfc.nasa.gov/691/cosmicice/interstellar.html 
http://www.astro.uni-koeln.de/cdms/molecules 
atom count