It turns out that dark matter just happens to be located in halos around the galaxies, which keeps the galaxies from collapsing from their own gravity.
As physicists learn more about dark matter and dark energy, they are realizing that there have been several eras since the Big Bang. In the beginning radiation was dominant and in the end the dark energy or vacuum energy will dominate. Right now, for a cosmologically brief period, matter (including dark matter) is of the same magnitude as dark energy. The "matter era" is fine-tuned and there is only a 1% chance that a random observer would be lucky enough to have matter and dark energy of the same order of magnitude. Cosmologists call this the Coincidence Scandal or problem.
http://nedwww.ipac.caltech.edu/level5/March04/Carroll/Carroll2_3.html
http://adsabs.harvard.edu/abs/2001astro.ph..5317D
Here is yet another example. During the first second of the Big Bang a little patch was expanded exponentially by just the right amount of "inflation" to give us this universe.
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