system
Given the following post, respond as if you're commenting on Reddit:
human
If stars in a galaxy increase in velocity the further they are from the center, why do galaxies have spirals? Wouldn't spiral arms indicate slower speeds as you approach the edges of a galaxy?

I understand dark matter plays a role here, keeping the galaxy spinning as one like a dinner plate.  Does this indicate the spirals form before enough dark matter appears?
gpt
Despite normal intuition when looking at a face on picture of a spiral galaxy, the spirals aren't exactly what they would seem. They are not concentrations of stars as you might assume and they have little to do with the motion of the stars. They are in fact density waves with in the material of the galaxy, namely gas and and dust. The slightly higher concentrations of particulate matter lead to the formation of larger, brighter stars. This leads to the characteristic spiral, not the motion/velocity of the stars.