The discovery of dark matter and the role of gravity in determining the mass of the universe will be explored in an interactive format making extensive use of computer simulations and independent projects. This is a "project based" course where students pose fundamental astrophysical questions and answer them in a series of projects designed to simulate a real research experience. Students work with data and bring their understanding of physics and astronomy to bear on interpreting and analyzing the data. In this class students design an experimental approach, write programs to analyze and visualize their data, develop interpretations and conclusions through class discussions and write a series of papers presenting their research. Each student participates regularly in class discussions and gives at least one oral presentation to the rest of the class. Many of the pedagogic goals of ASTRON 224 and 225 are identical; students are therefore advised to take only one of these courses.