In this book, we have, at length discussed dusty Shrouds of the Night — apart from their dust content, these masks also contain fi ery young blue stars, which obscure our view of what spiral galaxies actually appear like, beneath their Shrouds. However, we encounter an even more dramatic mask, which we term the mass mask. When galaxies are imaged behind their dust masks, they reveal the impressive backbones of spiral galaxies, as we have repeatedly seen — but it should always be remembered that the actual mass of stars in the disk of a spiral galaxy such as our Milky Way or the Andromeda Spiral is only a small fraction of its total mass. In the inner parts of a spiral galaxy, the stars do indeed contribute most of the mass, but as astronomers probe their disks further and further out in radius, another component of the mass dominates. This is the “missing mass” or enigmatic dark matter (discussed in greater detail later on in this chapter).
The first recognition of dark matter in spiral galaxies (as detected from the manner in which the galaxies rotate) was made in 1970. There is a fascinating historical interlude here yet again, this time not from the pen of Edwin Hubble or John Reynolds but from the pen of Princeton astrophysicist, the late John Bahcall.