Final answer:
Originally, the cause of the spiral arm structure in galaxies was thought to be differential rotation, but this was insufficient because it would lead to overly tight spirals over billions of years. Further research with supercomputer models has pointed to giant molecular clouds as initiators of the spiral arms, which are long-lasting due to gravitational forces.
Step-by-step explanation:
At first, differential rotation was thought to be the cause of the spiral arm structure in galaxies. This initial explanation suggested that the different rotational velocities at various distances from the galactic center would naturally lead to the material being stretched into elongated spiral features. However, differential rotation alone presented a problem because over the 13-billion-year history of the galaxy, it would have wound the spiral arms tighter and tighter, until their distinct structure would no longer be discernible.
Further investigation into the formation of spiral arms has shown that other processes are at work. Supercomputer calculations indicated that giant molecular clouds have significant gravitational influence to initiate spiral structures which then become self-perpetuating. These structures are maintained over time due to the concentration of matter in the arms, which provides enough gravitational force to keep the arms together. Bright, defined spiral arms appear to be long-lived features of galaxies.