Which discovery by astronomer vera rubin prove the existence of dark matter

When observing the Andromeda Galaxy, Vera Rubin discovered the material at the edges of  the spiral-shaped galaxy (like most other galaxies) were moving at the just as fast as those at the center (almost near the black hole). This challenged the Newtonian law of motion.  Therefore there had to be an unseen force that acted on these galaxies (dark matter) and space altogether.

Your answer is water because there is combined mass of water found on, under, and over the surface of a planet. your answer is water.