WMAP 5-Year Results Released | Cosmic Variance:Of course the human brain is 70% water, so we shouldn't feel entirely bad that most of what we know is only 5% of the universe.
...The WMAP folks have produced an elaborate cosmological parameters table that runs the numbers for different sets of assumptions (with and without spatial curvature, running spectral index, etc), and for different sets of data (not just WMAP but also supernovae, lensing, etc). Everything is basically consistent with a flat universe comprised of 72% vacuum energy, 23% dark matter, and 5% ordinary matter....
Here's what Wikipedia has on "vacuum energy":
... Two proposed forms for dark energy are the cosmological constant, a constant energy density filling space homogeneously, and scalar fields such as quintessence or moduli, dynamic quantities whose energy density can vary in time and space. In fact, contributions from scalar fields, which are constant in space, are usually also included in the cosmological constant. The cosmological constant is thought to arise from the vacuum energy...and
...Vacuum energy is an underlying background energy that exists in space even when devoid of matter ...So the recipe is basically definitional. Take away dark and ordinary matter and you get, by definition vacuum energy. Vacuum energy may arise largely from "smooth tension" (aka "dark energy"), but perhaps that's an interpretation of the results, not a statement of the results.
PS. Please note I'm not physicist in any form, I just like learning this stuff.
Update 3/8/2008: A little more detail on bad astronomy. Age - 13.73 billion years (middle aged in terms of a universe in which we can live). The average temperature of the universe is 2.725 degrees Kelvin -- just a smidgin above nothing.