Book excerpt: This dissertation explores the subatomic world with observations of neutron stars. Neutron stars are exotic and mysterious objects and are placed under the spotlight with current and planned investigations. In the coming decades, multimessenger (electromagnetic, neutrino, gravitational wave and cosmic ray) observations are expected to reveal the physical processes that underlie extreme astrophysical phenomena involving neutron stars such as compact star mergers and core-collapse supernovae. These observations could provide new insights about particle and nuclear physics under extreme conditions and physics beyond the standard model. Open questions concerning ab initio nuclear theory, dense quantum chromodynamics matter and the nature of dark matter are summarized. Following a brief review of neutron stars, the prospect of constraining neutron matter equation of state with future gravitational wave detections is examined. The rest of the thesis focuses on potential neutron star observables and constraints of a few emerging dark matter models. It is shown that dark baryons lighter than ~ 1.2 GeV are incompatible with observed massive pulsars. A scenario where effects of self-interacting dark matter may be detected via gravitational waves from binary neutron star mergers is studied. Finally, the closing chapter investigates impacts and potential observable signatures of a novel superfluid phase of matter arising from neutrino-portal scalars carrying lepton numbers in newly-born neutron stars.