Introduction

Dark matter is surely at the heart of modern cosmology. It undoubtedly pervades the Universe, unless we are being completely misled by diverse data sets, yet it has not been detected. The possible connection with proposed extensions of the Standard Model of particle physics, currently being searched for at accelerators, makes the identification of dark matter one of the highest priority goals in cosmology and particle physics. In this chapter we provide an introduction to the dark matter situation and an overview of the material presented in this book.

Dark matter has a venerable history (see e.g. ref. [742] for an historical account). One could even cite Solar System arguments for dark matter, including anomalies in the orbit of Uranus and the advance of Mercury's perihelion. One led to the discovery of a previously dark planet, Neptune, the other to a new theory of gravitation. Similar parallels may be drawn today. There are advocates of new theories of gravitation (see Chapter 6), who seek to dispense with dark matter, and there are observations of largescale structure, such as gravitational lensing, the cosmic web and the cosmic microwave background acoustic fluctuations, that are notoriously difficult to reproduce in the absence of a dominant component of cold dark matter (CDM) particles (we refer the interested reader to Chapter 4, which includes an introduction to gravitational lensing in the context of the CDM paradigm, and its potential to discriminate CDM from modified-gravity theories).