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.