If we instead weighed (technically, found the mass of) everything in the universe, we'd come up with around 75% of the universe is Hydrogen and 23% is Helium. And if you get that distinction, between number density (the former) and mass density (the latter, the normal concept of density), you are now at the same level as I was after a year of grad school.
Either counting by number (more common) or by mass, Hydrogen is the most abundant element in the universe, with Helium trailing far behind but making up most of the difference. H and He are the most important elements in the universe, whether we're in stars or giant gas clouds. Because of this, astronomers have a number of jokes about the chemicals in the universe.
- "The universe is made of Hydrogen, Helium, and everything else."
- "In astronomy there are three elements. We call Hydrogen X, Helium is Y, and everything else is Z."
- "If it's not Hydrogen or Helium, it's metal."
I'm not kidding you, we say these. We learn them in class!
Technically, cosmologists also worry about Lithium and Berylium, as mathematical and computer models can use their abundance today to determine details of how the Big Bang occured, but any other elements were created since the Big Bang. In the cores of stars of various mass fusion turns Hydrogen into everything upto and including Iron (our Sun will only get as far as Carbon though). Everything heavier than Iron was created in a supernova, as a massive star violently exploded. Our Sun will slowly expand as a Red Giant and gently shrug off its outer layers in a Planetary Nebula, but we're too small to supernova. Either way, the new chemicals created in a star are then scattered through out the rest of the nearby universe and eventually become part of new stars, new planets, and life as we know it.