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The standard hot DM candidate is massive neutrinos [18, 19, 20, 21], although other, more exotic theoretical possibilities have been suggested, such as a ``majoron'' of nonzero mass which is lighter than the lightest neutrino species, and into which all neutrinos decay. Neutrinos appeared to be an attractive DM candidate because of the measurement of an electron neutrino mass of about 30 eV in 1980 [22]. This coincided with the improving CMB limits on the primordial fluctuation amplitude, which forced Zel'dovich and other theorists to abandon the idea that all the dark matter could be made of ordinary baryonic matter. The version of HDM that they worked out in detail, with adiabatic Gaussian primordial fluctuations, became the prototype for the subsequent Omegam = 1 CDM theory.