Why do the “arms” of a spiral galaxy curve like a pinwheel? Shouldn’t they go straight out? Or even curve “backward”?
Angular momentum and gravity. There’s a cool animation here: Spiral Galaxy Animation
That said, classical physics doesn’t seem to paint the whole story. Much of the evidence for dark matter comes from the study of the motions of galaxies: Dark matter - Wikipedia
Sorry about the hijack, but why do galaxies rotate at all?
Because everything in the Universe rotates.
Galactic arms are density waves in the gas and dust. The rotation curves standingwave referenced above applies to stars, not to the spiral arms. This page has more on the density wave theory of sprial arms: Density wave theory
I have read and or saw something someplace that, the stars don’t rotate any faster at the center of the disk then at the outer parts.
So, in effect the galaxy is like a wheel and all parts are connected. Rather than like a liquid swirling in a drain.
Mass on the very large scale didn’t play nice with Newtonian physics, as particles in the small scale don’t either.
I think that’s where the Dark Matter/Energy idea was based partly upon, including looking for the lost mass of the universe.
As far as they why they rotate…it all just theory at the moment, i’m not sure if we really know if gravity is a product of Mass or Mass is a product of Gravity.
Meaning, Gravity could just be holes in space that attracts Matter into it, and the conservation of energy creates a spin as atoms are attracted to the Gravity Well.
Just a thought…
…:dubious:
No, if it were like a wheel, the parts at the rim would rotate faster than the parts at the center.
You’re correct that the fact that stars near the center are going at about the same speed as those near the rim is one of the main lines of evidence for dark matter. But I’m not sure what you’re talking about with gravity creating spin: In addition to energy being conserved, angular momentum (which is a way of measuring spin) is also conserved.
Yes, and that fact makes my head spin.
If they didn’t rotate there would be nothing to stop their own gravity collapsing them into a huge ball of matter, which would then become a black hole. Or to put it another way, galaxies that don’t rotate can’t exist [for long, anyway]. Rotation is all that keeps the stars and other galactic matter apart.
Imagine the conditions that would have to pertain for a random collection of stardust to coagulate into stars with no rotational component at all. Every speck would have to fall directly into the centre of gravity of an existing body, and each of those bodies would have to fall to each other or to larger accumulations directly to their centre of gravity, and so on. Vanishingly unlikely.
Thanks all, for your responses.
Are you sure you’ve got your cause and effect straight there? 
I recently saw the “Gravity” episode of “The Universe” Series on TV. Forget whether it was TLC, Discovery, or whatever. Either way, they were discussing gravity waves, and had an animation of a gravity wave superimposed over the image of a spiral galaxy.
I think the concept they were trying to get across is that gravity waves are present, and are the reason why certain regions of space (like galaxy arms) happen to be densely populated with matter while other regions (in between galaxies) are not so much.
OK, that makes sense. Thank you!