Description: In the past ten years we have completed a task set out by the ancients — we have weighed the universe. However, instead of simply finding out how much matter there is, we have found that what we typically think of as matter — things like protons and neutrons, charmed quarks, muons and gluons, stars and galaxies — make up only about 4 percent of the mass of the universe. And we have no idea what the other stuff is (well, we always have ideas...) Then, while trying to measure how the expansion of the universe is slowing down, we have found that the expansion is actually getting faster and faster. Whatever is causing this apparently makes up about 75 percent of the energy density of the universe! Just when we thought we were making progress sorting it all out, it now seems that we have no idea what most of the universe is made of, and only a superficial understanding of how it works fundamentally. That's a good time to be a physicist!
