With the recent discovery of the Higgs boson at the Large Hadron Col-
lider, the mechanism through which fundamental particles acquire mass in
the Standard Model of particle physics is now complete. However, the vast
majority of the visible mass of the universe resides in protons and neutrons
which are not fundamental, but composite particles of the quarks and glu-
ons whose interactions are described by Quantum Chromodynamics (QCD).
These strong interactions are responsible for 99% of the proton and neutron
masses, and therefore these bound states of quarks and gluons provide an
ideal laboratory to study QCD and elucidate our understanding of visible
matter in the universe. To that end, one of the primary goals of the STAR
experiment at the Relativistic Heavy Ion Collider is to use spin as a unique
probe to unravel the internal structure and the QCD dynamics of the nucleon
by studying high-energy polarized proton collisions. In this talk, I will dis-
cuss what we have learned about the origin of the proton's spin, emphasizing
recent developments in gluon and antiquark polarization.