Final answer:
Fritz Zwicky was the first to notice the high speeds of galaxies within clusters in the 1930s, leading to the idea of dark matter due to the discrepancy between observable mass and gravitational effects.
Step-by-step explanation:
The first person who noticed the surprisingly high speeds of galaxies within clusters was the Swiss-born American astronomer Fritz Zwicky in the 1930s. Confirmed by subsequent observations of other astronomers such as Vera Rubin, Zwicky's measurements indicated that stars within galaxies moved at velocities that did not decrease with the square root of the distance from the galaxy's center as expected. Instead, the velocity curve was almost flat, suggesting the presence of a significant amount of matter in the galactic halo, which led to the hypothesis of dark matter to explain this discrepancy between observable matter and actual gravitational effects.