It’s difficult enough, from inside, to determine in detail what the spiral structure of our galaxy is, but progress has been made recently. It’s also been determined that a nearby dwarf galaxy has passed through our galaxy at least twice in the past, and should have had a large effect on the current spiral structure. Now there is also a simulation that attempts to determine the effects in more detail.
A dwarf galaxy called Sagittarius can be credited with giving the Milky Way its signature spiral arms.
Sagittarius struck our galaxy some 1.9 billion years ago. It then looped over the galactic “north pole” and struck again about 900 million years ago. It is heading back right now, on course for a third clash in 10 million years or so.
These impacts must have had a considerable influence on the Milky Way but the effects have been hard to calculate because of uncertainties over the amount of dark matter in Sagittarius.