Dark Matter in the Universe

Abstract

What is the universe made of? What kind of matter is com­ monest, how much is there and how is it distributed? These ques­ tions, always a focus of cosmology, have become even more intriguing over the past few years as evidence has piled up to support the proposition that most of the mass in the universe is dark-invisible to any existing tele­ scope or other observational device­ and new developments in both high­ energy physics and astrophysics have made possible new predictions of the makeup and distribution of this possi­ bly exotic form of matter. There is already overwhelming evi­ dence that the visible matter within galaxies may account for less than 10 percent of the galaxies' actual mass: the rest, not yet directly detectable by observers on the earth, is probably dis­ tributed within and around each gal­ axy. Theoretical considerations now suggest this may be only the tip of the cosmic "iceberg" of dark matter: much greater amounts of dark matter may be distributed throughout the uni­ verse, perhaps in configurations en­ tirely independent of the distribution of galaxies. It may be that this mass can be accounted for only by the exis­ tence of new kinds of matter. The question of dark matter-how much of it there is, how it is distributed and what it is made of-is intimately linked to questions about the overall structure and evolution of the uni­ verse: because dark matter is probably the dominant form of mass in the uni­ verse, it must have affected the evolu­ tion of the features observable today. Questions of structure in turn depend for their answers on a deep bond that has formed between macro physics and microphysics, the bodies of knowledge that respectively describe interactions on the largest scale (that of the uni­ verse as a whole) and the smallest scale (that of the fundamental particles that make up all matter).