wrote:
It could also be fairly normal gas dynamics: densification and
rarification in a rotating body of gas and dust. You get the pile-ups
at the spiral arms and rarification between them.

The outer edges "drag" behind because they have lower orbital
velocities than material closer to the galactic core.

No, that doesn't add up.
(What you say is correct - but it isn't the explanation
of the spiral arms.)

Spirals seems to look more or less the same for billions
of years; the arms doesn't wind up as they would do if the reason
was as you say.
The matter density in a spiral galaxy isn't so different in the arms
and between the arms as a visible light picture makes it appear.
The spiral arms are moving through the matter of the galaxy.
What is seen as bright spiral arms is the star forming regions where
there are a lot of very bright blue giants (o and B stars). These only
live for few millions of years, so they are only present in the star
forming regions. In other words - star forming regions are bright.
The theory goes something along these lines:
The spiral arms are kind of pressure waves travelling through
the matter (gas and dust) of the galaxy. The compression of the gas
trigger the star formation, which in turn drives the the pressure wave on.

I think a lot of people are working of computer simulations
of this - which has been an enigma for a long time.

(Because it has been known for a long time that your rather - sorry -
naive explanation doesn't work.

Paul