  In 1900 it was believed that almost 100% of the mass of the Universe resided
in stars. Now, in the year 2000, such stars (and cold gas) are known to account
for only ~1% its mass. The remaining mass of the Universe is thought to reside
in hot baryons (~3%), cold dark matter (~30%) and dark energy (~66%). The
present paper traces the evolution of our thinking about the density of the
Universe during the Twentieth Century, with special emphasis on the of the
discovery of cold dark matter.
