What do the universe comprise of?

One of the greatest mysteries in physics today is trying to figure out all of the stuff in the universe which we can't see. In fact, the results of the WMAP survey has indicated that the majority of the universe is stuff that we can't see: dark matter and dark energy.
This graphic from 2008 shows the results from the WMAP survey in a pie chart, both "Today" and "13.7 Billion Years Ago" (when the universe was only 380,000 years old). This starkly shows the amazing evolution of matter in our universe over time. Here is the description of this data from the WMAP website:

WMAP data reveals that its contents include 4.6% atoms, the building blocks of stars and planets. Dark matter comprises 23% of the universe. This matter, different from atoms, does not emit or absorb light. It has only been detected indirectly by its gravity. 72% of the universe, is composed of "dark energy", that acts as a sort of an anti-gravity. This energy, distinct from dark matter, is responsible for the present-day acceleration of the universal expansion. WMAP data is accurate to two digits, so the total of these numbers is not 100%. This reflects the current limits of WMAP's ability to define Dark Matter and Dark Energy.
Of course, the "13.7 Billion Years Ago" data is based on some serious theoretical extrapolations back in time, so this is our best estimate based on current theories. It's possible that some of these theories will be modified in the years to come and the distribution of matter in the early universe was very different from what we now believe.
Of course, for that matter, we may someday realize that we're wrong about dark matter and dark energy, and even our understanding of the current universe could be wrong. That's always a possibility in science, and part of the reason why it's so interesting. For now, at least, this is the theory that most cosmologists believe has merit.