B. Absorption due to interstellar dust. Throughout the plane of our galaxy, there is a great deal of dust which obscures our view when we look anywhere along the Milky Way. This general obscuration prevents us from seeing any very distant objects such as other galaxies. Within this general pattern of dust certain more dense concentrations stand out. Thus stars in the front of the dust cloud are unaffected by it, while stars behind the dust cloud are reddened and have their apparent brightness dimmed.
Some clouds are so thick and dense that no stars can shine through.
Thus the stars seen on such a cloud are all in front of the cloud. The rift is visible at night to the naked eye as a dark band in the middle of the Milky Way. We conclude that this is due to absorption by dust clouds with most of the visible stars being between us and the dust. Sometimes the very dense absorbing dust clouds are clearly separated from anything else. They may well have very sharp edges.