A nearby dwarf galaxy, Sagittarius, has delivered two "punches" to our Milky Way galaxy. These impacts are what gave the Milky Way its spiraling arms. And we may take another hit soon.
At 10,000 light years across, Sagittarius is a small galaxy just a tenth the size of the Milky Way. It's also not very bright, and because it was obscured by the center of our galaxy, it wasn't discovered until 1994. Still, Sagittarius is loaded up with dark matter, and that has made its presence felt.
A recent study published in Nature reveals that Sagittarius smashed into the Milky Way twice in the last two billion years. Those impacts pushed stars outwards. Chris Purcell, lead author of the study, explains:
"When all that dark matter first smacked into the Milky Way, 80 percent to 90 percent of it was stripped off. That first impact triggered instabilities that were amplified, and quickly formed spiral arms and associated ring-like structures in the outskirts of our galaxy."
A tough hit for the Milky Way, but you should see the other guy. Sagittarius is literally getting ripped apart by the gravitational forces of the Milky Way. Before it dissolves, it may have another run at us, though. It's on course to hit the Milky Way one more time. We have another 10 million years to brace ourselves.