That 30 per cent of visible matter has remained invisible is not so wellknown problem related to dark matter. It is now identified and assigned to the network of filaments in intergalactic space. Reader can consult the popular article "Researchers find last of universe's missing ordinary matter" (see this). The article "Observations of the missing baryons in the warmhot intergalactic medium" by Nicastro et al (see this) describes the finding at technical level. Note that warmhot refers to the temperature range 10^{5}10^{6} K.
In TGD framework one can interpret the filament network as as a signature of flux tubes/cosmic string network to which one can assign dark matter and dark energy. The interpretation could be that the "invisible visible" matter emerges from the network of cosmic strings as part of dark energy is transformed to ordinary matter. This is TGD variant of inflationary scenario with inflaton vacuum energy replaced with cosmic strings/flux tubes carrying dark energy and matter.
This inspires more detailed speculations about prestellar physics according to TGD. The questions are following. What preceded the formation of stellar cores? What heated the matter to the needed temperatures? The TGD inspired proposal is that it was dark nuclear physics (see the article Cold fusion, low energy nuclear reactions, or dark nuclear synthesis?). Dark nuclei with h_{eff}=n× h_{0} were formed first and these decayed to ordinary nuclei or dark nuclei with smaller value of h_{eff}=n× h_{0} and heated the matter so that ordinary nuclear fusion became possible.
Remark: h_{0} is the minimal value of h_{eff}. The best guess is that ordinary Planck constant equals to h=6h_{0} (see this and this).
 The temperature of the recently detected missing baryonic matter is around 10^{6} K and roughly 1/10:th of the temperature 10^{7} K at solar core. This serves as a valuable guideline.
I already earlier realized that the temperature at solar core, where fusion occurs happens to be same as the estimated temperature for the binding energy of dark nuclei identified as dark proton sequences with dark nucleon size equal to electron size. The estimate is obtained by scaling down the typical nuclear binding energy for low mass nuclei by the ratio 2^{11} of sizes of ordinary and dark nuclear (electron/proton mass ratio, dark proton has same size as ordinary electron). This led to the idea that nuclear fusion in the solar core creates first dark nuclei, which then decay to ordinary nuclei and liberate essentially all of nuclear binding energy. After that ordinary nuclear fusion at resulting high enough temperature would take the lead.
 Dark nuclear strings can correspond to several values of h_{eff}=n× h_{0} with size scale scaled up by n. pAdic length scales L(k)= 2^{(k151)/2}L(151), L(151)≈ 10 nm, define favoured values of n as integers in good approximation proportional to 2^{k/2}. The binding energy scales for dark nuclei is inversely proportional to 1/n (to the inverse of the padic length scale). Could 10^{6} K correspond to a padic length scale k=137  atomic length scale of 1 Angstrom?
Could dark cold fusion start at this temperature and first give rise to "prenuclear physics generating dark nuclei as dark proton sequences and with dark nuclear binding energy about . 1 keV with dark nuclei decaying to k=127 dark nuclei with binding energy about 1 keV, and lead to heating of the matter and eventually to cold fusion at k=127 and after than the ordinary fusion? Also the values intermediate in the range [137,127] can be considered as intermediate steps. Note that also k=131 is prime.
 Interestingly, the temperature at solar corona is about 1 million degrees and by factor 140150 hotter than the inner solar surface. The heating of solar corona has remained a mystery and the obvious question is whether dark nuclear fusion giving rise to "prenuclear" fusion for k=137 generates the energy needed.
 If this picture makes sense, the standard views about the nuclear history of astrophysical objects stating that the nuclei in stars come from the nuclei from supernovas would change radically. Even planetary cores might be formed by a sequence of dark nuclear fusions ending with ordinary fusion and the iron in the Earth's core could be an outcome of dark nuclear fusion. The temperature at Earth's core is about 6× 10^{3} K. This corresponds to k=151 in reasonable approximation.
Remark: What is amusing that the earlier fractal analogy of Earth as cell would make sense in the sense that k=151 corresponds to the padic length scale of cell membrane.
I have also considered the possibility that dark nuclear fusion could have provided metabolic energy for prebiotic lifeforms in underground oceans of Earth and that life came to the surface in Cambrian explosion (see this). The proposal would solve the henegg question which came first: metabolism or genetic code since dark proton sequences provide a realization of genetic code (see this).
 One can imagine also a longer sequence of padic length scales starting at lower temperatures and and longer padic length scales characterized by integer k for which prime values are the primary candidates. k=139 corresponding to T=.5× 10^{6} K is one possibility. For k= 149 and k=151 (thicknesses for the lipid layer of the cell membrane and cell membrane) one would have T ≈ 2× 10^{4} K and T ≈ 10^{4} K  roughly the temperature at the surface of Sun and biologically important energies E= 2 eV of red light and E=1 eV of infrared light (quite recently it was found that also IR light can serve as metabolic energy in photosynthesis).
Could dark nuclear fusion process occur at the surface of the Sun? Could one image that the sequence of dark phase transitions proceeding to opposite directions as: k=137 ← 139 ← 149 ← 151→ 149→ 139→ 137→ 131→ 127 between dark nuclear physics corresponding to padic length scales L(k) takes place as one proceeds from the surface of the Sun upwards to solar corona and downwards to the core. Of course, also other values of k can be considered: k:s in this sequence are primes: the ends of the warmhot temperature range 10^{5}10^{6} corresponds roughly to k=143 = 13× 11 and k=137.
For TGD view about "cold fusion" and for comments about its possible role on star formation see the chapter
Cold fusion again or the article Cold fusion, low energy nuclear reactions, or dark nuclear synthesis?.
