Final answer:
Galactic rotation curves and motions within galaxy clusters provide evidence for dark matter, an invisible substance that contributes significantly more mass to galaxies than visible matter and greatly influences their structure and evolution.
Step-by-step explanation:
Observations of galactic rotation curves and motions within galaxy clusters have provided significant evidence for the existence of dark matter. The stars at the outer edges of galaxies, such as the Milky Way and the Triangulum galaxy, rotate with velocities that cannot be explained solely by the gravitational force of the visible matter—stars, gas, and dust. Instead, their constant orbital speeds suggest the presence of an additional, invisible mass, extending well beyond the luminous regions.
This unseen mass, termed dark matter, is believed to make up about ten times more of the galaxies' total mass than the luminous matter. It plays a pivotal role in the structure and evolution of galaxies and is essential for explaining the strong gravitational effects observed, which prevent galaxies from being torn apart by their rapid rotation. Notably, the existence of dark matter is supported not just by the rotation curves, but also by observations of gravitational lensing, X-ray emissions from galaxy clusters, and the dynamics of galaxies within these clusters.
The quest to understand dark matter encompasses identifying its composition, which has not yet been directly detected and may involve particles beyond our current understanding. It is a central mystery in the field of astrophysics that impacts our comprehension of the entire universe's structure and makeup.