  We explore the irreducible cosmological implications of a singlet real scalar
field. Our focus is on theories with an approximate and spontaneously broken
$\mathbb{Z}_2$ symmetry where quasi-stable domain walls can form at early
times. This seemingly simple framework bears a wealth of phenomenological
implications that can be tackled by means of different cosmological and
astrophysical probes. We elucidate the connection between domain wall dynamics
and the production of dark matter and gravitational waves. In particular, we
identify three main benchmark scenarios. The gravitational wave signal observed
by pulsar timing arrays can be generated by the domain walls if the mass of the
singlet is $m_s \sim\,$PeV. For lower masses, but with $m_s \gtrsim 10\,$GeV,
scalars produced in the annihilation of the domain walls can be dark matter
with a distinctive feature in their power spectrum. Finally, the thermal bath
provides an unavoidable source of unstable scalars via the freeze-in mechanism
whose subsequent decays can be tested by their imprints on cosmological and
terrestrial observables.
