  As a hypothetical topological defect in the geometry of spacetime, vortex
strings play a crucial role in shaping the clusters of galaxies that exist
today, and their distinct features can provide observable clues about the early
universe's evolution. A key feature of vortex strings is that they can interact
with Weyl fermionic modes and support topological chiral-anomaly states with
massless dispersions at the core of strings. To date, despite many attempts to
detect vortex strings in astrophysics or to emulate them in artificially
created systems, observation of these topological vortex-string chiral modes
remains experimentally elusive. Here we report the experimental observation of
such vortex-string chiral modes using a metamaterial system. This is
implemented by inhomogeneous perturbation of a Yang-monopole phononic
metamaterial. The measured linear dispersion and modal profiles confirm the
existence of topological modes bound to and propagating along the vortex string
with the chiral anomaly. Our work not only provides a platform for studying
diverse cosmic topological defects in astrophysics but also offers intriguing
device applications as topological fibres in signal processing and
communication techniques.
