I came across this link. In it the author speculates that in the outer region of a galaxy, the spiral arms overtake the stars, and vice versa for the inner region. That means there must be a middle region where the speed of the arms and the stars are similar. Since the stars there are big, blue and short-lived, they must die quickly. But because no other stars or clouds are moving into this region because of the same speed, it will turn dark quite fast in cosmic terms.
The result is, every spiral galaxy should have a distinctive dark, round disk of 'nothingness' somewhere between the central core and the outer rim. Looks like it's not the case. How can we explain this discrepancy?