What unexpected observation regarding outer stars led to the dark matter conclusion?
Answer
The stars were moving too fast based on visible matter.
Astronomers observed the rotational speeds of stars and gas clouds in the outer regions of galaxies, far from the visible central bulge. When calculating the gravitational pull expected solely from the luminous matter—the light emitted by stars and gas visible through telescopes—the calculated attraction was insufficient to hold these objects in their measured orbits. The orbital velocity of these outer stars was significantly higher than predicted, meaning they possessed enough velocity to escape the visible galaxy's gravitational pull, yet they remained tightly bound, pointing to an unseen mass component.
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