What effect does the presence of dark matter have on galaxy rotation curves?

The presence of dark matter significantly impacts galaxy rotation curves by causing them to flatten out at greater distances from the galaxy's center. In observations of spiral galaxies, it is noted that the rotation speeds of stars and gas remain constant at large radii from the center, rather than decreasing as expected if only visible matter were present.

According to Newtonian dynamics, if the mass of a galaxy were primarily composed of stars and gas, the velocity of stars orbiting further from the center would diminish, much like how planets behave in our solar system. However, the flat rotation curves suggest there is additional unseen mass contributing to the gravitational pull, which is attributed to dark matter. This unseen mass extends beyond the visible galactic structure, leading to a consistent orbital velocity for stars far from the galactic center, and resulting in the characteristic flattening of the rotation curve.

Thus, dark matter plays a critical role in maintaining the speeds of stars in the outer regions of galaxies, leading to the distinctive shape of the observed rotation curves.