From Lee Billings at Scientific American:
Whatever dark matter is, it is not accounted for in the Standard Model of particle physics, a thoroughly-tested “theory of almost everything” forged in the 1970s that explains all known particles and all known forces other than gravity. Find the identity of dark matter and you illuminate a new path forward to a deeper understanding of the universe—at least, that is what physicists hope
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“The desire is for dark matter to not only exist but also to solve other outstanding problems of the Standard Model,” says Jesse Thaler, a physicist at Massachusetts Institute of Technology. “Not every new discovery can be a revelation like the Higgs, where afterward theories suddenly fit together much better. Sometimes new particles just make you say, ‘Who ordered that?’ Do we live in a universe where each discovery leads to deeper, more fundamental insights or do we live in one where some parts have rhyme and reason but others don’t? Dark matter offers either possibility.”More.
See also: New class of galaxy mainly dark matter
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Of note: “some fine-tuning is required to explain the observations”
Besides Dark matter being fine tuned and having a hand in sculpting the Milky Way into its beautiful spiral shape, Scientific American also refers to the ‘Invisible Hand’ of Dark Energy, (which is fine tuned to within 1 part to 10^120), as also having a hand in shaping our galaxy into its spiral shape.
Here is the view from “Beyond Einstein: non-local physics” by Brian Fraser (2015):
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Einstein recognized that a “static” Universe (the accepted view in the early 1900s) could not be a stable one. The Universe would eventually collapse due to the action of gravitation. To counter that problem, Einstein introduced the “cosmological constant” to General Relativity in 1917. It represents what is now viewed as “dark energy” which keeps the Universe from collapsing. However, it was recognized that its inclusion still did not lead to a static Universe, because the equilibrium was unstable. If stars moved closer, the gravitational force would increase, moving closer still. If stars moved farther apart, then the gravitational effect would be lessened, and “dark energy” would more readily move them even farther apart. The whole situation was unstable, and to this day the cosmological constant is still regarded as an “outstanding theoretical challenge” in cosmology.
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The view that is gaining currency today is that space itself expands or is “emergent” (new spatial units are being generated by some unknown process). It is like time, in that it progresses. But it progresses in three dimensions, and we call that an expansion.
Opposing the expansion is gravitation, which is centered on an object (planet, star, galaxy). We interpret the resulting motions in terms of forces, the cosmological expansion force, which is not affected by distance, and the gravitational force, which has a 1/d^2 dependence. Because of this, there is necessarily a distance where the forces are at equilibrium . . . . For stars it is apparently a few light years, and for galaxies it is apparently a few million light years. Inside this distance, objects come together, and outside this distance, objects move apart. This is the “beyond Einstein” view that reconciles the issues of stability and instability. It explains why globular clusters are stable, even though they do not rotate sufficiently to keep them from collapsing. It explains why stars are separated by light years, but not by light weeks. It may explain some of the problems in calculating the Hubble constant, because the “constant” would be dependent on the location from which the observations are made (a large versus small galaxy).
Gravitation seems to have three regions. Gravitational force near a star starts out strong but declines rapidly with distance (the 1/d^2 region). At the gravipause, gravitation is still present, but falls off less rapidly (the 1/d^1 region, or “Hubble space” as it could be called). Beyond that, quantization causes the gravitation to disappear completely (the 1/d^0 region, where it does not decrease at all, because there isn’t any).
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The free 22 page paper can be downloaded from: http://scripturalphysics.org/4.....stein.html The .html file gives a link to the .pdf file but the former has additional information, and many more links and insights.