ABSTRACT
The density of luminous matter is thought to comprise less than 1 percent of the total density of all forms of matter and energy put together. The remaining 99 percent or more consists of dark matter which, while not seen directly, is inferred to exist from its gravitational influence on luminous matter as well as the geometry of the Universe. This chapter outlines what is known about dark matter in general, and how it is that we know it exists when it does not shine. Observation and experiment over the past few years have increasingly suggested that there are, in fact, four distinct categories of dark matter, three of which imply new physics beyond the existing standard model of particle interactions. The chapter provides a brief overview of the arguments for all four parts of nature’s ‘dark side’: baryonic dark matter, cold dark matter, hot dark matter, and vacuum energy.
