Gravitational theory, whether Newton’s theory of gravitation or Einstein’s general theory of relativity, explains, among other things, the motion of celestial objects. Several times throughout history, discrepancies between the theory’s predictions and the actual motion of these objects have been observed. One of the common attempts to account for this discrepancy is by means of something called “dark matter” [1] (not to be confused with dark energy, a different phenomenon that will not be discussed here): a mysterious substance that cannot be seen.
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The first use of the word “dark” in this context is attributed to Lord Kelvin, who wrote that “many of our stars, perhaps a great majority of them, may be dark bodies.” [2]. He said this in reference to the gap between the speed at which the stars in our galaxy should orbit its center, according to Newton’s law of gravitation, the estimated mass of that center, and the actual orbital speed.
The first person to use the term “dark matter” was the French physicist Henri Poincaré in 1906, in an article entitled “The Milky Way and the Theory of Gases” [2] written in response to Lord Kelvin. In this article, Poincaré wrote that because the observations are close enough to the theoretical results, there is no need for dark matter.
Since the 1920s, Einstein’s general theory of relativity has replaced Newton’s theory as the scientific framework that explains gravity. Although this theory accounts for the motion of celestial objects better than the Newtonian one, it too does not match all observations.
One of the most significant discrepancies was discovered in the 1960s and 1970s [2], when the American astronomer Vera Rubin used instrumentation developed by her colleague Kent Ford to observe spiral galaxies. Because most of the stars in such galaxies are concentrated around the galactic center, the hypothesis was that most of the galaxy’s mass is located there as well. This suggests that the rotational speed of stars around the galactic center should decrease the farther a star is from the center. Rubin’s observations showed that this is not the case; in fact, the stars’ velocities increase, leading to the conclusion that the galaxy’s mass is not concentrated at its center. On the other hand, most of the mass we can see is indeed near the center, implying that a large portion of the mass is invisible—namely, it is composed of dark matter. According to current estimates, the amount of mass that must reside in dark matter is more than five times the amount of visible mass [1].
The technical definition of dark matter is complex. For our purposes, we will review a few of its properties: first, it must have mass. Second, it must be something that cannot be seen. This means that it cannot emit or reflect electromagnetic radiation, not only in the visible range but across the entire (very broad) spectrum used by different telescopes. This property makes it extremely difficult to detect, because the vast majority of existing detectors rely on electromagnetic interaction—the very interaction that dark matter engages in only very weakly, if at all. This may be the reason that, although there are many candidates for dark matter—very cold stars, small black holes, Weakly Interacting Massive Particles (WIMPs), and others—we still do not know what dark matter is, or even whether it exists at all.
Some contend that there is no need for dark matter, but rather for a correction to the theories that describe gravity, both because dark matter has not yet been discovered and because no theory explains the behavior of dark matter. When, for example, we wish to determine the spatial distribution of dark matter, we observe an astronomical object such as a galaxy or a galaxy cluster and then add dark matter in such a way as to explain the discrepancy between the motion of the object’s stars and the motion predicted by the theory of relativity. This procedure has to be carried out separately for each object [3].
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English editing: Elee Shimshoni
