‘Questions for the 21st century’ looks at current trends in particle physics research. Up to 90% of the matter in the universe remains undetected, and many theories exist to explain what this ‘dark matter’ is. One theory is supersymmetry, which postulates that there is a unity between fermions (quarks and leptons) and bosons. Despite the lack of evidence, the theory solves a lot of problems in unifying quantum and particle physics. Alternatively, dark matter could be due to neutrinos spontaneously changing between generations. The Higgs boson may explain the fundamental nature of mass. Scientists also look for spontaneous appearances or disappearances, as this may suggest the existence of alternate dimensions.