  Dark matter, $X$, may be generated by new physics at the TeV scale during an
early matter-dominated (MD) era that ends at temperature $T_R \ll {\rm TeV}$.
Compared to the conventional radiation-dominated (RD) results, yields from both
Freeze-Out and Freeze-In processes are greatly suppressed by dilution from
entropy production, making Freeze-Out less plausible while allowing successful
Freeze-In with a much larger coupling strength. Freeze-In is typically
dominated by the decay of a particle $B$ of the thermal bath, $B \rightarrow
X$. For a large fraction of the relevant cosmological parameter space, the
decay rate required to produce the observed dark matter abundance leads to
displaced signals at LHC and future colliders, for any $m_X$ in the range ${\rm
keV} < m_X < m_B$ and for values of $m_B$ accessible to these colliders. This
result applies whether the early MD era arises after conventional inflation,
when $T_R$ is the usual reheat temperature, or is a generic MD era with an
alternative origin. In the former case, if $m_X$ is sufficiently large to be
measured from kinematics, the reheat temperature $T_R$ can be extracted. Our
result is independent of the particular particle physics implementation of $B
\rightarrow X$, and can occur via any operator of dimension less than 8 (4) for
a post-inflation (general MD) cosmology. An interesting example is provided by
DFS axion theories with TeV-scale supersymmetry and axino dark matter of mass
GeV to TeV, which is typically overproduced in a conventional RD cosmology. If
$B$ is the higgsino, $\tilde h$, Higgs, W and Z particles appear at the
displaced decays, $\tilde h \rightarrow h \tilde a, Z \tilde a$ and $\tilde
h^\pm \rightarrow W^\pm \tilde a$. The scale of axion physics, $f$, is
predicted to be in the range $(3\times10^8 - 10^{12})$ GeV and, over much of
this range, can be extracted from the decay length.
