This week's announcement of the discovery of the Higgs boson – the so-called God particle – was hailed as one of the great breakthroughs of the 21st century, explaining some of the fundamental physics of the universe. Yet in many ways the achievement has only highlighted how much we still do not know. The coming years will see humankind embark on new missions that will seek to advance our understanding: both into the limitless depths of space and the subatomic world within. Here are four questions that still vex science.