Phenethylamine, dopamine and norepinephrine.

While I agree with the answers given, it is an interesting question. It presupposes as fact that scientists on Earth can truly prove the composition of the entire Universe when we have never "touched" anything beyond our own solar system (and we haven't yet "touched" everything in our own solar system; where is that rock from Pluto??). While I understand the science that leads us to believe the answer is Hydrogen, including the use of some pretty advanced telescopes based on the entire spectrum of light and beyond, I just do not yet buy that we truly know everything there is to know about the Universe, including the most abundant element. In fact, the question presupposes that we know every element in the Universe. One only has to look back in our own scientific history to realize that 50 years ago, many of the current elements were not known. 200 years from now, who knows what we will know, and who knows what we will not know!

The most abundant element in the universe is hydrogen, which makes up about 3/4 of all matter! Helium makes up most of the remaining 25%. Oxygen is the third most abundant element in the universe. All of the other elements are relatively rare. If you would like to read more on the topic, here is a good article to read: http://chemistry.about.com/cs/howthingswork/f/blabundant.htm

Hydrogen.