<blockquote id="quote"><font size="2" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by Rudolf</i>
<br />Are there any other indications that 'stuff' like dark matter can exist (outside the galaxy rotation problem)? Obviously there could be a lot of plain dark normal matter that is not visible as well.<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">Even before the rotation problem was known, we had problems accounting for all the mass needed to bind galaxy clusters together. The same for superclusters. The larger the scale, the greater the discrepancy between seen and unseen matter (assuming the Newtonian law is still in effect).
Normally, to hide matter, it needs to be as clumped and compact as possible. However, if the mass is big enough to make stars, then thermonuclear fusion creates radiation that makes the mass visible. But if the mass is broken into tiny bits such as gas and dust, that is easily seen as absorption lines in all light passing through it.
So theory has focused on the two mass zones where detection would be hardest: MACHOs and WIMPs. MACHOs (Massive Compact Halo Objects) are planet-sized objects that would be hard to see directly. They have been searched for by looking for gravitational lens effects in the Large and Small Magellanic Clouds. Such objects between here and there would be detected by lensing stars in the two satellite galaxies. But there were not enough of them by far to explain the missing dark matter. WIMPs are Weakly Interacting Massive Particles -- presumably some as-yet-undiscovered quantum particle with mass and enough abundance to make a difference. Again, searches have been mostly fruitless, or at least insufficient to convince astronomers that the dark matter problem is nearing a solution.
The most succssful hypothesis so far is MOND or Modified Newtonian Dynamics, which hypothesizes a minimum acceleration -- a deviation from the Newtonian law of gravity. That explains dynamics without dark matter. It has been robust in its ability to explain phenomena, even ones not known when the hypothesis was formulated. But it has not gained much acceptance because (1) it would undermine the Big Bang; and (2) it is ad hoc, lacking a good physical justification.
MM offers an alternate interpretation and physical justification for a MOND-like mechanism. However, as MM has been further developed lately, it has become increasingly apparent that elysium plays a greater role in galaxy dynamics than I have heretofore appreciated. When I have time, I plan a complete rethink of large scale forces from first principles. -|Tom|-
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