Yes, thank you. You've given me a list of deep space anomalies for which astronomers and physicists have yet to find definitive explanations. Do any of these help us to establish definitive and measurable properties of dark matter?
Hot dark matter[edit]
Main article: Hot dark matter
Hot dark matter consists of particles that have a free-streaming length much larger than that of a proto-galaxy.
An example of hot dark matter is already known: the neutrino. Neutrinos were discovered quite separately from the search for dark matter, and long before it seriously began: they were first postulated in 1930, and first detected in 1956. Neutrinos have a very small mass: at least 100,000 times less massive than an electron. Other than gravity, neutrinos only interact with normal matter via the weak force making them very difficult to detect (the weak force only works over a small distance, thus a neutrino will only trigger a weak force event if it hits a nucleus directly head-on). This would make them 'weakly interacting light particles' (WILPs), as opposed to cold dark matter's theoretical candidates, the weakly interacting massive particles (WIMPs).
There are three different known flavors of neutrinos (i.e., the electron, muon, and tau neutrinos), and their masses are slightly different. The resolution to the solar neutrino problem demonstrated that these three types of neutrinos actually change and oscillate from one flavor to the others and back as they are in-flight. It is hard to determine an exact upper bound on the collective average mass of the three neutrinos (let alone a mass for any of the three individually). For example, if the average neutrino mass were chosen to be over 50 eV/c2 (which is still less than 1/10,000th of the mass of an electron), just by the sheer number of them in the universe, the universe would collapse due to their mass. So other observations have served to estimate an upper-bound for the neutrino mass. Using cosmic microwave background data and other methods, the current conclusion is that their average mass probably does not exceed 0.3 eV/c2 Thus, the normal forms of neutrinos cannot be responsible for the measured dark matter component from cosmology.[95]
Hot dark matter was popular for a time in the early 1980s, but it suffers from a severe problem: because all galaxy-size density fluctuations get washed out by free-streaming, the first objects that can form are huge supercluster-size pancakes, which then were theorised somehow to fragment into galaxies. Deep-field observations clearly show that galaxies formed at early times, with clusters and superclusters forming later as galaxies clump together, so any model dominated by hot dark matter is seriously in conflict with observations.
