  Measurements of the primordial element abundances provide us with an
important probe of our universe's early thermal history, allowing us to
constrain the expansion rate and composition of our universe as early as $\sim
1 \, {\rm s}$ after the Big Bang. Prior to this time, we have essentially no
empirical information on which to base any such claims. In this paper, we
imagine a future time in which we have not only detected the particles that
make up the dark matter, but have measured their mass and annihilation cross
section with reasonable precision. In analogy to the light element abundances,
the dark matter abundance in this scenario could be used to study and constrain
the expansion rate and composition of our universe at the time of dark matter
freeze out, which for a standard thermal relic occurs at $T_f \sim
m_{\chi}/20$, corresponding to $t \sim 4 \times 10^{-10} \, {\rm s} \times
({\rm TeV}/m_{\chi})^2$, many orders of magnitude prior to the onset of Big
Bang nucleosynthesis. As examples, we consider how such measurements could be
used to constrain scenarios which feature exotic forms of radiation or matter,
a ultralight scalar, or modifications to gravity, each of which have the
potential to be much more powerfully probed with dark matter than with the
light element abundances.
