What phenomenon invalidates the idea that visible matter alone holds the Milky Way together?
Answer
Stars far from the center maintain relatively flat orbital speeds.
Observations show that stars located far from the Milky Way's core maintain orbital speeds that remain relatively flat, or nearly constant, as their distance from the center increases. If the galaxy's mass were primarily derived only from the visible matter (stars, gas, and dust), orbital dynamics would predict that stars farther out should slow down significantly, following Keplerian dynamics similar to how outer planets orbit the Sun. The fact that these distant stars maintain high, consistent speeds indicates that a much larger gravitational influence, one not accounted for by visible mass, must be present throughout the outer regions of the galaxy.
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