What role does **gravity** play within structures like the Local Group?

The mutual attraction of members overrides the general expansion of space, keeping the structure bound.

The Local Group, which includes the Milky Way, Andromeda (M31), and Triangulum (M33), serves as a prime example of local gravitational dominance. While the universe exhibits large-scale expansion—the Hubble flow—gravity dictates structure formation and motion on smaller scales. Within gravitationally bound systems like the Local Group, the attractive force between members is significantly stronger than the outward push of expanding spacetime. Consequently, the structure itself is not expanding internally; instead, its members interact dynamically, leading to mergers, like the predicted collision between the Milky Way and Andromeda, rather than separation.