Abstract: A few basic considerations on Dark Matter, Dark Energy, Gravitation, and how science proceeds. For once, nearly no statement here is in any way controversial… (OK, except a few snide remarks on the motivations of some revered physicists, and some tenets of physics requiring experimental proof, rather than blind acceptance…)
Deep mine experiments look for WIMPs (Weakly Interacting Massive Particles). In Europe and the USA, the world’s two largest elementary particle accelerators have targeted their beams towards detectors in said mines, through hundreds of Kilometers of Earth’s mantle. Claims have been made for tentative events. It’s all very tentative, although tantalizing.
The idea for the WIMP search is a bit the same as with neutrino detection: bury deep underground, so that only WIMPs should be left.
Neutrinos are traditionally supposed to have mass zero (neutrino means the small neutral one, an Italian neologism invented by Enrico Fermi, Nobel prize winner, refugee from Mussolini, and scientific head of the nuclear bomb project in the USA; the concept was from Wolfgang Pauli, to explain the continuous spectrum of beta decay). But it turns out that neutrinos have mass, and they oscillate between types. A whole slew of experiments will try to know more soon. So neutrinos are a type of WIMPs, except that they are very light.
It has been known since 1933 (by the Swiss astronomer Zwicky, a supernova specialist, discoverer of the concept of neutron star) that galaxies and clusters of galaxies are missing mass by a factor of at least ten, if one uses straight Newtonian gravity on the observed motions of galaxies or their disks.
Now Newtonian gravity is the first order of the modern ("Einstein’s") theory of gravitation. Both theories differ only at very high speeds or fields. Hence the observed discrepancy would mean that our theory of gravitation is false. A class of modification of Newtonian theory was proposed (MOND = MOdified Newton Dynamics, where the gravitational attraction is the same for high acceleration, proportional to the inverse of the square of the distance, but slips into simply the inverse of the distance for weak accelerations).
But the way science proceeds is to milk a theory to death, until it dies. The Newton-Einstein theory of gravitation is correct in the Earth’s neighborhood, as far as we can see. So the reflex of experimentalists was to look for missing mass under the form of WIMPs. The search was on well before it became obvious neutrino had mass. Now that later fact is a game changer, since supernovas emit copious quantities of neutrinos. Indeed there is less of a missing mass now (maybe only a factor of five instead of ten…)
The apparent Dark Matter is concentrated in strange ways: halos, lobes…
Apparent Dark Matter Ring. Located in the "Fish" galactic cluster, 5 billion light-years away, and 2,6 million light-years across. Gravitational lensing can be seen (by looking carefully) and exhibits the enormous mass of the Dark Matter halo.
Please remark that in today’s physics, photons have mass zero. All other particles have some mass. The mass zero of the photon has been turned into religion by Relativity theory, but, ultimately, it ought to be considered an experimental fact, to be continually verified. If photons did have a mass, that would be much more of a game changer. It would most probably imply that gravitons have mass too, so gravity would have to be recomputed…
However, for reasons of logical completeness, Pauli (again!) noticed that Quantum Field Theory would work well if and only if each particle had a symmetric partner across the fermion-boson line. This is called SUSY ("SUperSYmmetry"). That would give plenty of WIMPs. So the search is on. 2010 will bring on line plenty of experiments worldwide, and the LHC in Geneva will ramp up in power.
As the preceding indicates, Dark Matter could be something radically new (new WIMPs), or simply explained: say by massive neutrinos (?).
Or then may be gravity was not as we extrapolated it to be, from around our little blue and white spaceship.
But the real mystery is DARK ENERGY. That, if confirmed, is way out of imagined physics (although it could be claimed that the ad hoc "cosmological inflation" used to explain the homogeneity of the Big Bang was just such a possible prediction!). The universe is expanding, as if there was out there a mysterious anti-gravity. There is no mechanism to explain this (although it can be written down in Einstein’s gravitation equation by re-introducing a scalar term, the cosmological constant, Einstein had introduced to make the universe static, before Hubble discovered the expansion, leading an opportunistic Einstein to declare the cosmological constant was "his greatest mistake").
In conclusion, we are extremely far from a final "theory of everything", contrarily to what some physicists have claimed with profit motivated outrageous naivety (there is great profit in books and fame). Profit is most often a bad adviser to the sharpest thinking (something profit obsessed American regressive economists fail to integrate).
Verily, in 25 centuries or so of official physics history, I do not know of one period when so much has been officially not known, and blatantly darkly mysterious. Anything could happen. For example, there is not one force theory, but two, and they do not agree conceptually. The explanatory scheme in gravitation (no force, just inertia), is completely different from the explanatory scheme for forces in Quantum Field Theory (whatever that mystery wrapped in an enigma, shrouded by immense Lagrangians, renormalized by reality turns out to want to say).