Possible Duplicate: Dark matter references I have recently read about dark matte
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Question
Possible Duplicate:
Dark matter references
I have recently read about dark matter and dark energy, and why physicists think it must exist (dark matter: mass of galaxies are far bigger than expected, its gravitational effect on visible matter; dark energy: the force that causes the accelerated expansion of the universe).
I however don't find the case for dark matter or dark energy particularly strong, and I have a couple of questions regarding this:
Why do so many physicists accept the existence of dark matter and dark energy?
Are there any good alternative theories to explain these phenomena without the use of dark energy and/or matter?
Is the field of study regarding dark matter and dark energy (and possible alternative explanations) 'alive' or not so much? I still am young, but I find this interesting and if it has good prospects (like for example string theory does) I might like to work in it in the future.
Explanation / Answer
First of all, dark matter and dark energy, despite their naming, are two very different concepts. We don't really have any good reason to group them together, other than the fact that both represent things we don't understand. Thus they are not necessarily backed by the same sets of evidence.
Why we believe these things exist
As it happens though, some of the strongest evidence for both comes from the cosmic microwave background. Basically, whatever "stuff" there is in the universe will have an effect on the temperature and polarization fluctuations in this radiation, which was emitted some few hundred thousand years after the Big Bang.
The best all-sky map of this radiation is made by the WMAP satellite, and every couple years they release several papers with the analysis of the data. For instance, here is the 2011 paper focusing on the cosmological parameters. They basically just feed all the data into an enormous statistical program to find the most likely values for a large set of parameters, including the amount of "normal" baryonic matter ?b, the amount of cold dark matter ?c, the amount of dark energy ??, and the dark energy equation of state parameter w. There is a lot that can be said as to what the effects of these parameters are, but ultimately you just cannot explain the CMB without having dark energy and dark matter.
Alternate theories for dark matter
Now "cold dark matter" (the CDM of the ?CDM model) means massive particles interacting via gravity and the weak force but not via electromagnetism and that were non-relativistic even at the time of recombination when the CMB was released. Just plain old "dark matter" refers to any gravitating mass that doesn't have much of an electromagnetic signal. The galaxy rotation curves you refer to were some of the first dark matter evidence, and indeed they could be explained by assuming a large number of quiescent black holes or star-less planets or dust that we just missed for one reason or another. These alternate theories could also, with enough manipulation, explain the bullet cluster, where the gravitational mass found with lensing maps is clearly not collocated with the baryonic mass in hot, X-ray emitting intracluster gas. However, microlensing surveys tend to rule out the first two, and we think we have a good handle on dust dynamics. Something more exotic is called for. There is a diminishing community in support of MOND - modified Newtonian dynamics - which postulates long-range deviations from the inverse-square law in gravity. However, the bullet cluster, together with very precise Solar system data, makes this theory difficult to get working.
Add to this the very nice "WIMP miracle" (no good wiki page there - sorry), which is suggestive of dark matter being new types of particles. "WIMP" stands for "weakly interacting massive particle," and the "miracle" is this: If you assume there is a species of particle X whose only appreciable interaction is annihilation with its antiparticle X