Behind the mighty visible Universe that envelops us at night, hides something different and much more impressive. Most astronomers today believe that ninety-five percent of the contents of the universe are invisible. What we see in everyday life and in beautiful pictures from the Hubble Telescope is probably only five percent of what is hidden in space. The rest is dark matter and energy - that part of reality that we have only just begun to understand. So what are these dark invisible substances? And how can we be so sure of their existence? Jostein Rieser Christiansen (b. 1981) is a Norwegian astrophysicist and science popularizer.