"In the photographs he had seen of spiral galaxies, he had always assumed that it was the stars that were distributed in spiral arms, separated by relatively empty space. But now it was perfectly clear that the distribution of the stars in the disc was more or less regular, and that it was rivers of dust and gas that created the dark areas which divided them." (James Blish, Mission To The Heart Stars, London, 1980, p. 35)
This is "...clear..." to Jack because he is viewing not a photograph but an evolving map. He sees that, when all the dust and gas has become stars, the Milky Way will be a simple disc like many other galaxies. But is this right? I thought that the dark areas between spiral arms were empty space, not dust and gas concealing stars in those areas?
I will leave this question there for the time being.