NASA Hubble Spies Galactic Traffic Jam

NGC 3887 is one of many galaxies in our universe with spiral arms, just like our own Milky Way. Until the 1960s, the behavior of spiral arms was an astronomical puzzle. The arms emanate from a spinning core and "should" therefore become wound up ever more tightly over time. Instead, they move more slowly, like an interstellar traffic jam. via NASA http://www.nasa.gov/image-feature/goddard/2020/hubble-spies-galactic-traffic-jam