how can this be explained?
Space Tiger answered this very nicely when someone asked about the solar system being on a 'plane'. As matter with angular momentum comes together, a disk shape is the most natural organization. The spiral aspect is probably (as my guess) the result of matter arriving at irregular intervals and attaching itself to the nearest gravity source. As more stuff accumulates, the gravitational attraction in that area increases and attracts more matter, and thus it escalates.
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