Editing Neutrino (section)

=== Big Bang ===
{{Main|Cosmic neutrino background}}
It is thought that, just like the cosmic microwave background radiation leftover from the Big Bang, there is a background of low-energy neutrinos in our Universe. In the 1980s it was proposed that these may be the explanation for the [[dark matter]] thought to exist in the universe. Neutrinos have one important advantage over most other dark matter candidates: They are known to exist. This idea also has serious problems.

From particle experiments, it is known that neutrinos are very light. This means that they easily move at speeds close to the [[speed of light]]. For this reason, dark matter made from neutrinos is termed "[[hot dark matter]]". The problem is that being fast moving, the neutrinos would tend to have spread out evenly in the [[universe]] before cosmological expansion made them cold enough to congregate in clumps. This would cause the part of dark matter made of neutrinos to be smeared out and unable to cause the large [[galaxy|galactic]] structures that we see.

These same galaxies and [[galaxy groups and clusters|groups of galaxies]] appear to be surrounded by dark matter that is not fast enough to escape from those galaxies. Presumably this matter provided the gravitational nucleus for [[galaxy formation and evolution|formation]]. This implies that neutrinos cannot make up a significant part of the total amount of dark matter.

From cosmological arguments, relic background neutrinos are estimated to have density of 56&nbsp;of&nbsp;each type per cubic centimeter and temperature {{val|1.9|u=K}} ({{val|1.7|e=-4|u=eV}}) if they are massless, much colder if their mass exceeds {{val|0.001|u=eV/c2}}. Although their density is quite high, they have not yet been observed in the laboratory, as their energy is below thresholds of most detection methods, and due to extremely low neutrino interaction cross-sections at sub-eV energies. In contrast, [[boron-8]] solar neutrinos—which are emitted with a higher energy—have been detected definitively despite having a space density that is lower than that of relic neutrinos by some six [[orders of magnitude]].