{"id":1108,"date":"2017-03-12T05:36:24","date_gmt":"2017-03-12T03:36:24","guid":{"rendered":"https:\/\/www.lbscience.org\/en\/2025\/09\/23\/%d7%94%d7%97%d7%95%d7%9e%d7%a8-%d7%94%d7%90%d7%a4%d7%9c-%d7%94%d7%95%d7%90-%d7%a9%d7%9d-%d7%9b%d7%95%d7%9c%d7%9c-%d7%9c%d7%a8%d7%9b%d7%99%d7%91-%d7%9c%d7%90-%d7%9e%d7%95%d7%9b%d7%a8-%d7%a9%d7%9c\/"},"modified":"2025-10-11T04:24:49","modified_gmt":"2025-10-11T01:24:49","slug":"what-is-dark-matter","status":"publish","type":"post","link":"https:\/\/www.lbscience.org\/en\/2017\/03\/12\/what-is-dark-matter\/","title":{"rendered":"What is Dark Matter?"},"content":{"rendered":"

The first use of the word \u201cdark\u201d in this context is attributed to Lord Kelvin, who wrote that \u201cmany of our stars, perhaps a great majority of them, may be dark bodies.\u201d [2<\/a>]. 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\u2019s law of gravitation, the estimated mass of that center, and the actual orbital speed.<\/p>\n

The first person to use the term \u201cdark matter\u201d was the French physicist Henri Poincar\u00e9 in 1906, in an article entitled \u201cThe Milky Way and the Theory of Gases\u201d [2<\/a>] written in response to Lord Kelvin. In this article, Poincar\u00e9 wrote that because the observations are close enough to the theoretical results, there is no need for dark matter.<\/p>\n

Since the 1920s, Einstein\u2019s general theory of relativity has replaced Newton\u2019s 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.<\/p>\n

One of the most significant discrepancies was discovered in the 1960s and 1970s [2<\/a>], 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\u2019s 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\u2019s observations showed that this is not the case; in fact, the stars\u2019 velocities increase, leading to the conclusion that the galaxy\u2019s 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\u2014namely, 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<\/a>].<\/p>\n

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\u2014the 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\u2014very cold stars, small black holes, Weakly Interacting Massive Particles (WIMPs), and others\u2014we still do not know what dark matter is, or even whether it exists at all.<\/p>\n

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\u2019s stars and the motion predicted by the theory of relativity. This procedure has to be carried out separately for each object [3<\/a>].<\/p>\n

---<\/p>\n

Source link from the Facebook<\/a><\/p>\n

English editing: Elee Shimshoni<\/p>\n","protected":false},"excerpt":{"rendered":"

The first use of the word \u201cdark\u201d in this context is attributed to Lord Kelvin, who wrote that \u201cmany of our stars, perhaps a great majority of them, may be dark bodies.\u201d [2]. 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