The Nature of Dark Matter

By the end of 1970s most objections against the dark matter hypothesis were rejected. In particular, luminous populations of galaxies have found to have lower mass-to-luminosity ratio than expected previously, thus the presence of extra dark matter both in galaxies and clusters has been confirmed. However, the nature of dark matter and its purpose was not yet clear. Also it was not clear how to explain the Big Bang nucleosynthesis constraint on the low density of matter, and the smoothness of the Hubble flow. 

The Nature of Dark Matter Neutrino-dominated Universe 

Already in 1970s suggestions have been made that some sort of non-baryonic elementary particles may serve as candidates for dark matter particles. Gunn et al. (1978) considered heavy stable neutral leptons as possible candidates for dark matter particles, however in a later study Tremaine Gunn (1979) 

The search of dark matter can be illustrated with the words of Sherlock Holmes When you have eliminated the impossible, whatever remains, however improbable, must be the truth (cited by Binney Tremaine 1987). 

After my talk at the Caucasus Winter School Zeldovich offered me collaboration in the study of the universe. He was developing a theory of formation of galaxies (the pancake theory, Zeldovich 1970) an alternative whirl theory was suggested by Ozernoy (1971), and a third theory of hierarchical clustering by Peebles (1971). Zeldovich asked for our help in solving the question Can we find some observational evidence which can be used to discriminate between these theories  

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Abstract We discuss some of the astrophysical techniques to detect the presence and the nature of Dark Matter in large scale structures. We focus here on the cases of galaxies and galaxy clusters. 

The nature of Dark Matter, despite its overwhelming evidence, is still intangible. It is nonetheless appealing, in these respects, that some astrophysical features of galaxy clusters and of galaxies might give information on the fundamental properties of the DM particles. 

As we see, the emphasis of the conference was on the discussion of the physical nature of dark matter and its role in the formation of galaxies. These preliminary studies demonstrated that both suggested models for coronas had difficulties. It is very difficult to explain the physical properties of the stellar corona, also no fast-moving stars as possible candidates for stellar coronas were found. 

It was clear that by sole discussion only the presence and nature of dark matter cannot be solved, new data and more detailed studies were needed. 

And a more general question in Nature everything has its purpose. If 90 % of matter is dark, then this must have some purpose. What is the purpose of dark matter ... 

Evidence does suggest the presence of dark matter, whose nature is yet unknown. 

The search for weakly interacting particle, which may constitute the dark matter in the universe. The elucidation of the nature of this dark matter is one of the most pressing problems in astrophysics and in cosmology. 

Nowadays we hear a great deal about physicists ongoing effort to understand the nature of the universe s ultimate constituents. Numerous books are written about the physics of elementary particles, about the hypothetical objects known as superstrings, and about the dark matter that constitutes a large part of the universe s mass. Millions of words are written about attempts to probe their mysteries. 

The Earth is naturally cold, because of its participating more of the nature of the opaque and dark Primal Matter. This cold makes the body, heavier and denser and this density renders it less penetrable to Light, which is the principle of heat. It has been created in the midst of the waters, with which it is always mixed and the Creator seems to have made it dry on its surface, only to render it suitable for the abode of Vegetables and animals. 

The preceding interpretation rests heavily on the hypothesis that the dark matter consists primarily of the lightest N = l SUSY neutralinos. This is well motivated, but as has often been emphasized, the most compelling and elegant explanation of any natural phenomenon is often false. Of course, if accelerator evidence were found for SUSY, the odds in favour of a neutralino explanation of dark matter would be dramatically increased. 

Nothing is known about the nature of the energy component, which goes under the name of dark energy. Of the matter component, less than 2% is luminous, and no more than 20% is made of ordinary matter like protons, neutrons, and electrons. The rest of the matter component, more than 80% of the matter, is of an unknown form which we call non-baryonic. Finding the nature of non-baryonic matter is referred to as the non-baryonic dark matter problem. 

In the Type II category we put all hypothetical cold dark matter candidates that are neither Type la nor Type lb. Some of these candidates have been proposed for no other reason than to solve the dark matter problem. Others are examples of beautiful ideas and clever mechanisms that can provide good possibilities for non-baryonic dark matter, but in some way or another lack the completeness of the theoretical particle physics models of Types la and lb. Although Type II candidates are not studied as deeply as others, it may well be that eventually the question of the nature of cold dark matter might find its answer among them. 

Current cosmological data imply the existence of non-baryonic dark matter. We have discussed some of the most popular candidates and shown that none of the candidates known to exist, i.e. the active neutrinos, can be non-baryonic cold dark matter. Hence to explain the nature of cold dark matter we need to invoke hypothetical particles that have not been detected yet. Some of these hypothetical particles have been suggested for reasons different than the dark matter problem (such as sterile neutrinos, neutralinos, and axions), some others have been proposed mainly as a solution to the cold dark matter problem (e.g., self-interacting dark matter, WIMPZILLAs, etc.). Although most studies focus on the first category of candidates, especially neutralinos and axions, we should keep an open mind. 

All of the examples we have presented are without doubt simple, elegant, and compelling explanations for the nature of non-baryonic dark matter. As we ponder on which one of them is realized in Nature, we must remember the words of astrophysicist Thomas Gold (as quoted by Rocky Kolb) For every complex natural phenomenon there is a simple, elegant, compelling, wrong explanation."... 

The present state of knowledge about invisible dark matter indicates that it represents the predominant part (about 90%) of the total mass of the universe. It includes the so-called missing mass. Dark matter is not discernible by any kind of electromagnetic radiation in the region from y rays to radiowaves, but its gravitational effects on other kinds of matter are observable. For example, the rotation of spiral galaxies such as the Milky Way can only be explained if 90% of the matter is invisible in the sense mentioned above. The question of the nature of the dark mass is still open. Various possibilities are discussed matter different from that on the earth (no protons, neutrons and electrons), remnants of the big bang, neutrinos. However, the actual mass of neutrinos is still uncertain. 

There are many mysteries concerning the Milky Way, including the antimatter fountains, the nature of the energetic activity at its core, the unknown composition of the dark matter in the halo, and the uncertain process by which it formed. 

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