The flat rotation curves of spiral galaxies imply that the distribution of mass resembles a. the Solar System; most mass is concentrated in the center. b. a wheel; the density remains the same as the radius increases. c. the light distribution of the galaxy; a large concentration occurs in the middle, but significant mass exists quite far out. d. an invisible sphere much larger than the visible galaxy.

Spiral galaxies are one of the most fascinating structures in the universe. They are bright and beautiful, containing both young and old stars arranged in a distinctive spiral pattern. These galaxies include well-known examples like the Milky Way and Andromeda.

They feature a central bulge, which is a dense group of older stars, and a disk where the spiral arms extend. The spiral arms contain a mix of gas, dust, and many young, bright stars. Stars in spiral galaxies orbit the center of the galaxy.

Understanding spiral galaxies helps us learn how galaxies form and evolve. It also reveals the distribution of mass and the presence of mysterious dark matter.

Mass distribution within a galaxy can tell us a lot about its composition. In the case of spiral galaxies, you might expect most of the mass to be concentrated at the center, like the Solar System. However, observations show that this is not the case.

When astronomers measure the rotation curves of spiral galaxies, they find something surprising. The velocity of stars orbiting the galaxy center remains roughly constant, even far from the center. This phenomenon is described as a flat rotation curve.

If the mass distribution followed the light distribution (most mass at the center), the rotation speed would decrease with distance from the center. Instead, the flat rotation curve suggests a different mass distribution. This indicates that there must be a significant amount of unseen mass extending far beyond the visible boundaries of the galaxy.

The concept of dark matter arises from the observation of flat rotation curves in spiral galaxies. While visible matter, like stars and gas, is concentrated in the galaxy's central regions, flat rotation curves imply the presence of additional mass that cannot be seen.

Dark matter makes up this unseen majority. It does not emit, reflect, or absorb light, making it invisible to the current astronomical instruments. However, its gravitational effects are noticeable.

Through gravitational lensing, the rotation curves of galaxies, and the behavior of galaxy clusters, we infer that dark matter forms an extended halo around galaxies. This halo provides the additional gravitational pull necessary to maintain high rotational speeds of stars far from the galaxy center.

Dark matter continues to be one of the biggest mysteries in modern astrophysics. Understanding it could open the door to discovering new particles and forces in physics.