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the other thing to remember is that the arms you see in spiral galaxies are actually optical illusions brought about by the density waves moving through the galaxy. We see the arms because of the amount of star formation going on there, but after the star formation finishes and the wave moves on the dust and gas is still there. So in effect the areas between the arms is full of dust as well. And stars, but they are cooler yellow and red stars that live longer than the hot OB and A stars and not as easily visible to us.
At least that's my understanding.
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