  Three-dimensional (3D) topological states resemble truly localised,
particle-like objects in physical space. Among the richest such structures are
3D skyrmions and hopfions that realise integer topological numbers in their
configuration via homotopic mappings from real space to the hypersphere (sphere
in 4D space) or the 2D sphere. They have received tremendous attention as
exotic textures in particle physics, cosmology, superfluids, and many other
systems. Here we experimentally create and measure a topological 3D skyrmionic
hopfion in fully structured light. By simultaneously tailoring the polarization
and phase profile, our beam establishes the skyrmionic mapping by realising
every possible optical state in the propagation volume. The resulting light
field's Stokes parameters and phase are synthesised into a Hopf fibration
texture. We perform volumetric full-field reconstruction of the $\Pi_3$
mapping, measuring a quantised topological charge, or Skyrme number, of 0.945.
Such topological state control opens new avenues for 3D optical data encoding
and metrology. The Hopf characterisation of the optical hypersphere endows a
new perspective to topological optics, offering experimentally accessible
photonic analogues to the gamut of particle-like 3D topological textures, from
condensed matter to high-energy physics.
