Observations show that the dark matter in a galaxy surrounds the visible matter in a halo that is larger and more nearly spherical than the stars and gas that we see. The visible matter density is higher than the dark matter density near the centers of most galaxies, so the dark matter is not very important there. But it extends well beyond the stars and gas, so the outer parts of galaxies are essentially all dark matter. The result is represented in this "artist's impression" of a dark halo surrounding an almost edge-on disk galaxy. An image of the galaxy NGC 4216 from Frei and Gunn's color atlas has been embedded in a blue glow to represent the density of dark matter. In reality, the dark matter would be invisible; its properties could only be inferred indirectly by observing the motions of the stars and gas. But this artist's impression shows how the dark matter surrounds the visible galaxy.
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John Kormendy (firstname.lastname@example.org)