In the quiet outskirts of spiral galaxies, stars move at speeds that defy our familiar understanding of gravity. According to Kepler's laws, stars at the outer edges should slow to a gentle crawl, yet they keep pace with the crowded center instead. This flat rotation curve is a quiet, persistent anomaly. It points to an immense, unseen presence — a halo of dark matter surrounding the galaxy. The outer stars do not fly off into the dark because they are held by this invisible, gravitational pull.
Notice how the outer stars move rapidly through the dark, anchored the whole time by a gentle, unseen force.
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Dark Matter