A couple of years ago I started playing a game called Kerbal Space Program (KSP) and my understanding about how orbits actually work increased dramatically. Because of KSP I was looking at a picture of a galaxy with its spiral arms and something struck me as very odd. The stars closer in should orbit the galaxy center faster than the stars farther out and that would smear out these lanes of stars until they were no longer recognizable. Something weird is going on here!
So why do galaxies have arms? What's bunching these stars together like that?
I noticed that the center of galaxies are all smeared out and bulbous like I think that the rest should look like. So what if all that mass rotating around a common center is warping space around it, like the water draining down your sink, and this warping is what catching matter/stars into the lanes.