Professor John Womersley (STFC) gives the Particle Physics Christmas Lecture. In the past five years particle physicists have made major advances in understanding the nature of our universe – discovering the Higgs boson, and more recently detecting gravitational waves from a distant galaxy. Paradoxically we have also learned a lot more about what we don’t know: that the particles and forces we understand in ever greater detail make up only a small fraction of what’s in the cosmos, and that our theoretical prejudices about what remains to be discovered may have been very wrong. A new generation of ambitious experiments at accelerator laboratories, underground, and studying at the large scale structure of the universe will answer these questions – and surely open up others. I will also outline why it is essential that the country remains at the forefront of frontier research of this kind and how it contributes more broadly to society.