Dark matter is one of the most fundamental and perplexing issues of modern physics. Its presence is deduced from a straightforward application of Newton’s theory of gravity to astronomical systems whose dynamical motion should be simple to understand.
The success of Newton’s theory in describing the behavior of the solar system was one of the greatest achievements of the 18th century. Its subsequent use to deduce the presence of a previously unknown planet, Neptune, discovered in 1846, was the first demonstration of how minor departures from its predictions indicated additional mass.
The expectation in the early 20th century, as astronomical observations allowed more distance and larger celestial systems to be studied, was that galaxies and collections of galaxies should behave like larger solar systems, albeit more complicated. However, the reality was quite different. It is not a minor discrepancy, as led to the discovery of Neptune, but it is extreme. The stars at the edges of galaxies are not behaving at all like Pluto at the edge of the solar system. Instead of having a slower orbital speed, as expected and shown by Pluto, they have the same speed as those much further in. If Newton’s law is to be retained, there must be much more mass in the galaxy than can be seen, and it must be distributed out to large distances, beyond the visible extent of the galaxy. This unseen mass is called “dark matter,” and its presence was becoming widely accepted by the 1970s. Subsequently, many other types of astrophysical observations covering many other types of object were made that came to the same conclusions.
The ultimate realization was that the universe itself requires dark matter to explain how it developed the structures within it observed today. The current consensus is that one-fourth of the universe is dark matter, whereas only 1/20th is normal matter. This leaves the majority in some other form, and therein lies another mystery—“dark energy.”
The modern form of Newton’s laws is general relativity, due to Albert Einstein. This offers no help in solving the problem of dark matter because most of the systems involved are nonrelativistic and the solutions to the general theory of relativity (GR) reproduce Newtonian behavior. However, it would not be right to avoid mentioning the possibility of modifying Newton’s laws (and hence GR) in such a way as to change the nonrelativistic behavior to explain the way galaxies behave, but without changing the solar system dynamics. Although this is a minority concept, it is nonetheless surviving within the scientific community as an idea.
Understanding the nature of dark matter is one of the most intensely competitive research areas, and the solution will be of profound importance to astrophysics, cosmology, and fundamental physics. There is thus a huge “industry” of direct detection experiments predicated on the premise that there is a new particle species yet to be found, and which pervades the universe. There are also experiments searching for evidence of the decay of the particles via their annihilation products, and, finally, there are intense searches for newly formed unknown particles in collider experiments.
