Final answer:
The assumption that the mass of a galaxy is concentrated toward its center is reasonable because it is supported by observations of spiral galaxies, where the rotational velocity does not decrease with distance from the center. This suggests the presence of additional mass, known as dark matter. The concentration of mass toward the center is influenced by the presence of a supermassive black hole.
Step-by-step explanation:
The assumption that the mass of a galaxy is concentrated toward its center is a reasonable one. Spiral galaxies, such as the Milky Way, have a concentration of mass in their central regions due to the presence of a supermassive black hole. This central concentration of mass affects the rotational velocity of objects in the outer regions of the galaxy.
When astronomers measure the rotation speed of stars or gas in spiral galaxies, they observe that the rotational velocity does not decrease with increasing distance from the center. This observation suggests that there is additional mass (dark matter) beyond what is visible in the form of stars, gas, and dust that is causing the observed velocity. This discrepancy between the expected and observed rotational velocity supports the idea that the mass is concentrated toward the center.