The ‘strong’ force is the focus of a new field of particle physics, Quantum chromodynamics (QCD), which describes its interactions with matter. Specifically, QCD describes interactions between quarks (matter particles) and the gluons (strong force particles) that ‘glue’ them together to form so-called hadrons such as protons and neutrons. No quark has ever been observed in isolation, suggesting that quarks and gluons are permanently confined inside hadrons. QCD predicts that, at very high temperatures, the two may become deconfined to exist in a new state of matter called quark-gluon plasma (QGP). Heavy-ion experiments (using heavy metals) conducted at the Large Hadron Collider (LHC), the largest and most powerful particle collider ever built, are focused on producing and studying this extreme phase of matter. European researchers supported by funding of the ‘Heavy ion collisions at the LHC: Strong coupling techniques for high density QCD’ (HICLHC) project sought to provide theoretical interpretations of experimental results. Scientists studied lead-lead collisions at the LHC and electron-proton, proton-proton and deuteron-gold collisions from other laboratories. Given the broad nature of the experiments, investigators were able to produce a unified and consistent generalised description of high-energy phenomena. Researchers focused on the colour glass condensate (CGC), an extreme state of matter that may be the basis of the QGP. The HICLHC team provided an excellent theoretical description of experimental results on deuteron-gold collisions with easy-to-use routines. Scientists merged CGC theoretical tools with a Monte Carlo treatment to model the initial state of heavy ion collisions. Predictions regarding lead-lead collisions were in excellent agreement with experimental results later obtained. HICLHC project results provided important modelling tools made freely available on the web for the heavy ion community. The team also produced significant theoretical results concerning extreme states of matter that confirmed experimental results and provided a basis for predicting future outcomes and designing future experiments.