The outer parts of spiral galaxies rotate at high speeds due to the conservation of angular momentum. Angular momentum is a fundamental property of rotating objects, and it remains constant unless acted upon by an external torque.
When galaxies form, they undergo a process known as gravitational collapse. As the gas and dust within a galaxy collapse under their own gravity, they begin to rotate. Initially, the rotation is relatively slow. However, as the collapse continues and the galaxy's size decreases, its rotation speeds up due to the conservation of angular momentum.
As the collapsing gas and dust form a disk-shaped structure, called a protoplanetary disk or accretion disk, the conservation of angular momentum causes the rotation to become more rapid as the material moves closer to the center. This phenomenon is analogous to an ice skater spinning faster when they pull their arms inward.
The outer parts of spiral galaxies, including their spiral arms, are composed of stars, gas, and dust. These components inherit the rapid rotation from the initial collapse of the protoplanetary disk. Over time, the gravity of the galaxy's central bulge and the distribution of matter throughout the galaxy help maintain the rotation of the outer parts.
It's worth noting that the presence of dark matter also plays a significant role in determining the rotation curve of galaxies. Dark matter is an invisible form of matter that exerts gravitational forces on visible matter. Its distribution throughout a galaxy influences the rotation speed of the outer regions, causing them to rotate faster than what would be expected based solely on the visible matter. This discrepancy between observed rotation speeds and the predicted rotation based on visible matter is known as the "galaxy rotation problem" and is one of the pieces of evidence for the existence of dark matter.