Large galaxy surveys, like the Center for Astronomy (CfA)
redshift survey5 and the Sloan Digital Sky Survey6 performed
from the late 1970s through the early 2000s, were able to measure
the mass of the universe and found it to be far too low to
be consistent with a flat cosmos. Even including dark matter,7
the amount of mass in the universe seems to be about only
one-third of that needed. Given the compelling measurements
of the geometry of space, it was clear that there was
a mystery and the universe contained mass and energy that
had not been identified. Resolving this conundrum would
require a precise understanding of the expansion history of
the universe.