  We show that a nonstandard cosmological history with a period of early matter
domination driven by a sub-TeV visible-sector particle can arise rather
naturally. This scenario involves a long-lived standard model singlet that
acquires a thermal abundance at high temperatures from decays and inverse
decays of a parent particle with SM charge(s), and subsequently dominates the
energy density of the Universe as a frozen species. Entropy generation at the
end of early matter domination dilutes the abundance of dangerous relics (such
as gravitinos) by a factor as large as $10^4$. The scenario can accommodate the
correct dark matter relic abundance for cases with $\langle \sigma_{\rm ann} v
\rangle_{\rm f} \lessgtr 3 \times 10^{-26}$cm$^3$s$^{-1}$. More importantly,
the allowed parameter space can be directly probed by proposed searches for
neutral long-lived particles at the energy frontier, allowing us to use
particle physics experiments to reconstruct the cosmological history just prior
to big bang nucleosynthesis.
