Galactic rotation curves indicate the presence of unseen mass extending beyond visible matter.

In the 1970s, pioneering astronomer Vera Rubin and her colleagues observed that stars at the outer edges of spiral galaxies were orbiting much faster than predicted by the amount of visible matter alone. According to Newtonian dynamics, stars further from the galactic center should slow down, much like planets further from the sun. However, galactic rotation curves remained flat, implying a significant amount of invisible, gravitationally influential matter extending far beyond the luminous disk. This 'missing mass' provided one of the earliest and most compelling pieces of evidence for dark matter. This phenomenon suggests that galaxies are embedded within vast halos of dark matter, whose gravitational pull prevents the outer stars from flying off into intergalactic space. Without this invisible mass, galaxies as we know them would simply tear themselves apart.