Dark matter is believed to give galaxies its shape and prevent them from flying apart. Is dark matter present only on outer edges of the galaxy or is it present throughout the galaxy?
In order to explain the rotation curves of galaxies, the dark matter would have to be distributed throughout the galaxy and not only in the outer edges. More detail below:
The basic experimental results that lead to the discovery of dark matter is the rotation curve of galaxies. The rotation curve is basically a graph that plots speed of rotation (of material in the galaxy) at various distances from the center of the galaxy. We can theoretically calculate this rotation curve based on how much mass is encircled at each radius and using Newton's law of gravitation. When we went to do this calculation, we found that the theoretical results of the calculations didn't match the actual rotation curves of the galaxies. This led to the hypothesis that there is extra mass (dark matter) that isn't accounted for in the luminous matter of the galaxy. In order to explain the discrepancy of the calculated rotation curve to the real rotation curve, the dark matter would have to be pretty much distributed throughout the galaxy and not only at the outer edges.