Ibn al-Kawwa' asked 'Ali about the Milky Way. He said, "It is the water-trough (or loop of the bag) from which the heaven opens up flowing water."  

عَنِ الْمَجَرَّةِ قَالَ هُوَ شَرَجُ السَّمَاءِ وَمِنْهَا فُتِحَتِ السَّمَاءُ بِمَاءٍ مُنْهَمِرٍ  

Ibn 'Abbas said, "The rainbow is security for the people of the earth that they will not be drowned. The Milky Way is the door of the heavens and forms a furrow through it."  

عَنِ ابْنِ عَبَّاسٍ الْقَوْسُ أَمَانٌ لأَهْلِ الأَرْضِ مِنَ الْغَرَقِ وَالْمَجَرَّةُ بَابُ السَّمَاءِ الَّذِي تَنْشَقُّ مِنْهُ