I was just poking around to see if rift was a term or name used in astronomy, and there is an example of such (Wikipedia, of course!):
In astronomy, the Great Rift (sometimes called the Dark Side, Dark Rift, or, less commonly, Dark River) is a series of overlapping, non-luminous, molecular dust clouds that are located between the Solar System and the Sagittarius Arm of the Milky Way Galaxy at a distance of about 100 parsecs or about 300 light years (2×1015 miles or 3×1015 kilometers) from Earth. The clouds are estimated to contain about 1 million solar masses of plasma and dust.[1]
So it could be referring to similar dark patches. If you rotate the galaxy map, you do see dust clouds and dark bits that don't let as much starlight through.
There's also dark strips of low luminosity, unscoopable stars that divide the majority of main sequence stars.