  MATHUSLA is a proposed surface detector at CERN that would be able to observe
the decays of non-hadronic electrically neutral long-lived particles (LLPs)
with almost no background or trigger limitations. This would allow MATHUSLA to
probe sub-GeV to TeV masses and lifetimes up to $c\tau \sim 10^7~{\rm m}$.
MATHUSLA can play an important role in probing dark-matter scenarios involving
extended hidden sectors, where additional dark states often manifest as LLPs. A
prime example of such a scenario is furnished by the Dynamical Dark Matter
(DDM) framework, which intrinsically gives rise to large ensembles of dark
states exhibiting a broad range of masses and lifetimes. In this paper, we
examine the extent to which MATHUSLA can probe the DDM parameter space, and we
demonstrate that MATHUSLA may be capable of providing direct confirmation of
certain unique aspects of the DDM framework which might be difficult to probe
in other ways.
