system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How spiral arms form in galaxies

Shouldn't it be like stirring hot chocolate powder in a cup?
gpt
The speed of rotation of a galaxy is a bit weird to say the least.

The outermost edge rotates slowly, and as you make your way inwards, the rotation speed increases. The speed increases until the middle of the disc (im talking about the radius of the galaxy, not the diameter). From the middle of the radius to the centre of the galaxy, speed again decreases.

So what you have here is bits of matter (gas, dust, planetary systems etc) and stars which are held loosely together due to gravity. The matter bunched up into arms, due to the arrangement of the galactic magnetic fields.

Due to the uneven rotation speed, the middle of these structures gets "smeared" all over the place, as the faster middle overtakes the slow centre and the edge.

ps : Its a good thing we are living in a spiral galaxy, the other kinds of galaxies have virtually no heavy elements, hence, chances of intelligent life or even, higher forms of life is almost zero.