English: Animation that gives an overview of the eROSITA instrument. Very little is known about the Universe. The ingredients that make up four percent of its energy density – ‘normal’ material such as protons and neutrons – is only a very small part of the ‘Universe recipe’. What the other 96 percent is composed of remains a mystery. Today it is believed that 26 percent is Dark Matter. However, the largest share, estimated at 70 percent, is comprised of Dark Energy. To track this down, scientists must observe something unimaginably large and extremely hot. Galaxy clusters are composed of up to several thousand galaxies that move at different velocities within a common gravitational field. Inside, these strange structures are permeated by a thin, extremely hot gas that can be observed through its X-ray emissions. This is where eROSITA’s X-ray ‘eyes’ come into play. They allow to observe galaxy clusters and see how they move in the Universe, and above all, how fast they are travelling. We hope that this motion will tell us more about Dark Energy.