[PatrickHenry]

I am still going to disagree because as of now there is no way of knowing if you re-did those measurements on the same galaxy 1 million or 10 million years, etc down the road they would still be the same. In a spiral galaxy there probaby are way to many intangables (i.e. the way stars interact with each other) to determine if our "snapshot" measurement will always hold true.

Right. A million years from now, all those galaxies might even turn into pork chops or something. We don't really know. However ...

I believe that many galaxies, of various ages (judged by the composition of their stars) have been observed. There seems to be none of the rotational disparity you suggest. Thus the assumption that such things remain somewhat steady over time. But you're right. We don't really know.

[qam1]

Right. A million years from now, all those galaxies might even turn into pork chops or something. We don't really know. However ...

I believe that many galaxies, of various ages (judged by the composition of their stars) have been observed. There seems to be none of the rotational disparity you suggest. Thus the assumption that such things remain somewhat steady over time. But you're right. We don't really know.

Pork Chops? Come on now, Cut me some slack

I am not talking about things that are necessary age related, Things like

As the galaxy rotates do the spiral arms remain consistant or do they expand and contract?

Do Spiral arms remain consistant forever? i.e. Do new arms form while others disapate?

Do Eddies at times form during the rotation of galaxies?

At this time we don't and can't know if any of those factors or others we can't even image have any effects on our measurements. Until we do know for sure it seems kind of "Pre-mature" to invent this invisable matter that can do or be anything the observer wants.