Electromagnetizm. Nothing fancier than that is needed. It's as clear as day in some cases:
A good example is the spiral galaxy shown above. The arms, particularly the upper arm, show material being held in a straight line until some point at which the field breaks down, after which material very quickly trails away and dissipates. Gravity and inertia cannot do things like that.
We’ve found another “I Sing The Universe, Electric” person!
Obviously that is a huge galaxy. Is that a quasar on the lower right? The thing is so bright it is burning up the film/ccd.