Re: 'Dark Matter'---where is it?

"GH Diel" <ghdiel@xxxxxxxxxxxx> writes:

If dark matter constitutes the vast percentage of the universe,
why is it not found all around us?

If it exists, it almost certainly _is_ "all around us" --- but it must be
in some form that interacts only very weakly with "ordinary" matter.

For example, one proposed candidate for the "Dark Matter" that makes up
the galactic halo is _massive neutrinos_. Because these neutrinos would be
"viralized ("thermalized") within the galactic gravity well, they would be
relatively slow-moving (speeds on the order of typical stellar velocities,
on the order of a few hunderd kps), and hence would interact even more weakly
than nuclear-energy neutrinos, which already pass through "ordinary" matter
like ghosts. (Neutrinos can only interact with matter via the Weak and
Gravitational forces. To a first approximation, the neutrino Weak Interaction
cross-section scales as the square of the neutrino's kinetic energy; since
typical nuclear energies are on the order of MeV, whereas typical "halo"
neutrino energies may be expected to be more on the oder of eV, one may
expect that "halo" neutrinos will interact with "ordinary" matter about a
trillion times more weakly than the already ghostly weak interactactions
of nuclear-energy neutrinos.

Other candidates for "Dark Matter" are:

* The still-hypothetical "WIMPs" (Weakly Interacting Massive Particles),
which will be neutrino-like in the weakness of their interactions, but
extremely heavy by comparision. (One proposed candidate would be so-called
"Sterile" neutrinos that are not part of a weak isodoublet; other proposed
candidates are the fermionic "superpartners" of "ordinary" bosons,
e.g., photinos or gravitinos.)

* The equally hypothetical so-called "MACHOs" (MAssive Compact Halo Objects),
dense objects with masses presumed to be in the planetary to stellar range,
but composed of some unknown form of "non-baryonic" matter such as
"Planck-Mass relics" or "Strange Matter" or "Mirror Matter"
(AKA "Shadow Matter"); small "primordial" black holes left over
from before the "Nucleosynthesis Age" would also qualify. There is
enough empty space between the stars that such stellar-mass "MACHOs"
could easily outnumber "ordinary" stars by 10 to 1, and still not be
easily seen.

* "Axions," which are hypothetical pseudoscalar particles that were
proposed to explain why the "Strong" (or "Color") interaction
does not suffer from CP-violating "anomalies."

A final possibility that has been proposed are various exotic classes
of "topological defects" in the Higgs/gauge fields, such as "monopoles,"
"cosmic strings," "cosmic membranes," or "cosmic textures" ---
albeit monopoles are essentially ruled out at the galactic scale,
"cosmic strings" are ruled out except perhaps within the galactic core or
on supergalactic scales, and "membranes" and "textures" are firmly ruled out
below the supergalactic scale, so none of these are good candidates for
"halo" Dark Matter.


-- Gordon D. Pusch

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