  The ongoing quest for understanding nonequilibrium dynamics of complex
quantum systems underpins the foundation of statistical physics as well as the
development of quantum technology. Quantum many-body scarring has recently
opened a window into novel mechanisms for delaying the onset of thermalization
by preparing the system in special initial states, such as the $\mathbb{Z}_2$
state in a Rydberg atom system. Here we realize many-body scarring in a
Bose-Hubbard quantum simulator from previously unknown initial conditions such
as the unit-filling state. We develop a quantum-interference protocol for
measuring the entanglement entropy and demonstrate that scarring traps the
many-body system in a low-entropy subspace. Our work makes the resource of
scarring accessible to a broad class of ultracold-atom experiments, and it
allows one to explore the relation of scarring to constrained dynamics in
lattice gauge theories, Hilbert space fragmentation, and disorder-free
localization.
