... 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. ... 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. ... NASA Web Site Statements, Warnings, and Disclaimers . ... Publications with keywords: dark matter - spiral galaxy . Publications with words: dark matter - spiral galaxy . ...