5.1. 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) rejected this possibility. Cowsik & McClelland (1973), Szalay & Marx (1976) and Rees (1977) noticed that neutrinos can be considered as dark matter particles; and Chernin (1981) showed that, if dark matter is non-baryonic, then this helps to explain the paradox of small temperature fluctuations of the cosmic microwave background radiation. Density perturbations of non-baryonic dark matter start growing already during the radiation-dominated era whereas the growth of baryonic matter is damped by radiation. If non-baryonic dark matter dominates dynamically, the total density perturbations can have an amplitude of the order 10-3 at the recombination epoch, which is needed for the formation of the observed structure of the Universe. This problem was discussed in a conference in Tallinn in April 1981. Here all prominent Soviet cosmologists and particle physicists participated (this conference was probably the birth of the astro-particle physics). The central problem was the nature of dark matter. In the conference banquet Zeldovich hold an enthusiastic speech: "Observers work hard in sleepless nights to collect data; theorists interpret observations, are often in error, correct their errors and try again; and there are only very rare moments of clarification. Today it is one of such rare moments when we have a holy feeling of understanding the secrets of Nature." Non-baryonic dark matter is needed to start structure formation early enough. This example illustrates well the attitude of theorists to new observational discoveries - the Eddington's test: "No experimental result should be believed until confirmed by theory" (cited after Turner 2000). Now, finally, the presence of dark matter was accepted by leading theorists.
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).