Cosmological dark matter in the form of neutrinos with masses of up to a few electron volts is known as hot dark matter. In 1979-83, this appeared to be perhaps the most plausible dark matter candidate. Such HDM models of cosmological structure formation led to a top-down formation scenario, in which superclusters of galaxies are the first objects to form, with galaxies and clusters forming through a process of fragmentation. Such models were abandoned when it was realized that if galaxies form sufficiently early to agree with observations, their distribution would be much more inhomogeneous than it is observed to be. Since 1984, the most successful structure formation models have been those in which most of the mass in the universe is in the form of cold dark matter (CDM). But mixed models with both cold and hot dark matter (CHDM) were also proposed in 1984, although not investigated in detail until the early 1990s.
The recent atmospheric neutrino data from Super-Kamiokande provide strong evidence of neutrino oscillations and therefore of non-zero neutrino mass. These data imply a lower limit on the HDM (i.e., light neutrino) contribution to the cosmological density 0.001 - almost as much as that of all the stars in the centers of galaxies - and permit higher . The ``standard'' COBE-normalized critical-matter-density (i.e., m = 1) CDM model has too much power on small scales. It was discovered in 1992-95 that CDM with the addition of neutrinos with total mass of about 5 eV, corresponding to 0.2, results in a much improved fit to data on the nearby galaxy and cluster distribution. Indeed, the resulting Cold + Hot Dark Matter (CHDM) cosmological model is arguably the most successful m = 1 model for structure formation [1, 2, 3, 4].
However, other recent data have begun to make a convincing case for 0.3 m 0.5. In light of all these new data, several authors have considered whether cosmology still provides evidence favoring neutrino mass of a few eV in flat models with cosmological constant = 1 - m. The conclusion is that the possible improvement of the low-m flat (CHDM) cosmological models with the addition of light neutrinos appears to be rather limited, but that CHDM models with 0.1 may be consistent with presently available data. Data expected soon may permit detection of such a hot dark matter contribution, or alternatively provide stronger upper limits on and neutrino masses.