image credit: John Vickery and Jim Matthes/Adam Block/NOAO/AURA/NSF; image source; larger image
This is NGC 3198, a spiral galaxy. Astronomers originally assumed that most of the mass in this kind of galaxy was concentrated in the bright central region and, therefore, that the stars in the spiral arms moved in circular gravitational orbits--like the planets in the solar system. But when astronomers measured how the stars move, they found something quite different--see Rotation Curves for results for NGC 3198--and they were forced to the conclusion that much of the mass in galaxies does not emit light (thus the name "dark matter").
Imagine a universe where physicists understand only a small percentage of the matter in it--that's the situation today with the Missing Mass, one of the great unsolved questions in physics. For a discussion of the observations that led to this challenging situation, visit the Wikipedia entry Dark Matter.