From November 2002 until february 2007 I had a PhD position at the Netherlands Institute for Space Research (SRON). Here I did astronomical research on clusters of galaxies, the biggest structures in our universe. For this research I used space-based X-ray telescopes like XMM-Newton, Chandra and Astro-E2. The gas in clusters of galaxies can be so hot that it emits X-rays (about 10 million degrees!). These observations give a new insight in the structure of clusters and the universe as a whole.