Scientists tend to believe that the elementary structures underlying
the world we observe are ultimately simple and beautiful, even (or
especially) the structures we have not yet discovered. Still,
this basic elegance does not always manifest itself directly --
the universe we see is something of a mess.
This pie chart is a rather prosaic representation of a truly
impressive accomplishment: an inventory of
the relative amounts of the different substances comprising our
universe. Yellow is ordinary
matter -- atoms, molecules, dust,
stars, planets, both visible and invisible -- or what cosmologists
call "baryons" (since most of the mass of ordinary matter comes
from the protons and neutrons inside atomic nuclei, and protons
and neutrons are classified by particle physicists as baryons).
Baryons make up about five percent of the known universe (actually
closer to four percent, but let's not be picky). We know this
from a variety of independent measurements, including the
results of nucleosynthesis in the Big Bang, measurements of temperature
fluctuations in the cosmic microwave background, and (less
precisely) by direct detection. Everything we have ever seen is
only one-twentieth of everything there actually is.
