Final answer:
Galaxies within the Local Group move due to gravitational forces, including from the Virgo Supercluster mass, and the presence of dark matter is inferred from these movements.
Step-by-step explanation:
Galaxies within the Local Group move under the influence of gravitational forces. The Local Group, which includes the Milky Way and the Andromeda galaxy among its three dozen or so members, lies on the outskirts of the Virgo Supercluster. The mass at the center of the Virgo Cluster leads to a gravitational pull on the Local Group, causing it to move away from the Virgo Cluster at a slightly slower velocity than predicted by the Hubble law. This differential movement helps astronomers estimate the total mass in the Local Group, which includes a significant amount of dark matter.
Moreover, galaxies in clusters orbit around the cluster's center of mass, although these orbits take about a billion years and cannot be observed in their entirety. Still, by measuring the velocities of these galactic movements, astronomers can infer the cluster's total mass. Such estimations have revealed that the mass of visible galaxies is insufficient to hold the cluster together, suggesting the expansive presence of dark matter both within and between galaxies.