The origin of composite structure in the universe occurs at about 5 microseconds in the cosmological evolution: a phase transition from elementary quarks to composite hadrons, in particular "our familiar" protons and neutrons. We will have a look at the elementary theory of strong interaction, Quantum Chromodynamics (QCD), in order to understand the nature of this structure formation process. Due to the non-abelian colour charge of QCD, single colour carriers have infinite free energy if isolated in vacuum--quarks exhibit a "horror vacui". To confront the falling energy density in the big bang expansion (foreshading dilution toward vacuum density) the initial quark-gluon population has to undergo colour neutralisation, and make a quantum mechanical tunneling transition to neutral, on shell hadrons. Note that at energy density below the "critical" density of QCD, of about 1GeV per cubic Fermi (i.e. 1.6x10¹⁸ Kg per cubic meter!) a state of densely packed hadrons and hadronic resonances has lower free energy density than a partonic state. A "confining" phase transition occurs between these states, properties of which having recently been elucidated by ultrarelativistic nucleus-nucleus