The New Scientist has a great article on the discovery of some of the first characteristics we've learned about dark matter! I took an illustration from Scientific American and added in some dark matter visualizations. They are the blue bubble things.
Dark matter comes in clumps (bricks? discrete particles?) at least 1,000 light years across. Since we're just 28,000 light years from the Galactic Core, that means there are at most 28 bits of dark matter between us and the center of our galaxy. Giant Betelgeuse is about 560 light years away, double that, and you've got roughly the size of a dark matter brick. Does one bit of dark matter repel others, so they keep their distance? Do they slide around each other in the heavy gravity well at our galaxy's center? When or where do they begin to thin out?
Dark matter has gravity, which is the main reason we're sure it exists; there's not enough visible matter to hold galaxies together. Dark matter outweighs visible matter in our galaxy by a factor of about five! So each of those bricks weighs a heckuva lot!
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