Galaxies are fascinating not only for what is visible, but for what is invisible. The visible is dominated by millions of bright stars and dark dust , caught up in a gravitational swirl of spiral arms rotating about the center. Open clusters containing bright blue stars can be seen sprinkled along these spiral arms, while dark lanes of dense interstellar dust can be seen sprinkled between them. Less visible, but detectable, are billions of dim normal stars and vast tracts of interstellar gas , together wielding such high mass that they dominate the dynamics of the inner galaxy . Invisible are even greater amounts of matter in a form we don't yet know - pervasive dark matter needed to explain the motions of the visible in the outer galaxy.
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