When we measure how fast stars orbit far from the center of spiral galaxies, what unexpected pattern of speed tells us about the presence of unseen dark matter?

When we measure how fast stars and gas clouds orbit far from the center of spiral galaxies, the unexpected pattern of speed observed is that their orbital velocities remain remarkably constant, even at great distances from the galactic center. This observation is known as a "flat rotation curve." Based solely on the visible matter in a galaxy, which includes all the stars, gas, and dust that emit or reflect light, astronomers would expect a different pattern. According to standard gravitational physics, specifically Keplerian mechanics, objects orbiti....