A report in the journal Nature shows how the Milky Way's famous spiral arms are the product of a collision with the Sagittarius dwarf galaxy.  The model (see embedded video below) slammed past the Milky Way and much of it's dark matter was pulled off disrupting the stability of the galactic disc.

It's interesting to see the large streams of stars that are pulled from Sagittarius as it passes by - those loops (stars from both galaxies) have been pulled by gravity and the rotation of the larger galaxy into what are now the spiral arms the Milky Way is famous for.

The dwarf galaxy has whizzed past the Milky Way twice in the last 2 billion years and will do so again though the effects won't be as dramatic in subsequent passes as the dark matter has for the most part been stripped away from the smaller body.

Below is a computer model that the team uploaded to Youtube.