Summary: According to the currently accepted model in cosmology, our universe is made up of 5% of ordinary matter, 25% cold dark matter, and 70% dark energy. But what kind of entities are dark matter and dark energy? This chapter asks what the evidence for these entities is and which rival theories are currently available. This provides with an opportunity to explore a well-known philosophical problem known as under-determination of theory by evidence.
Comment: This Chapter could serve as an introduction to contemporary cosmology and particle physics or as an example to illustrate the problem of under-determination of theory by evidence. The chapter looks at alternative theories that explain the same experimental evidence without recourse to the hypothesis of dark matter and dark energy and discusses the rationale for choosing between rival research programs. Like the rest of the chapters in this book, it is a reading recommendable for undergraduate students. It is recommended to read it after Chapter 2 of the same book.