The Singularity on Instagram: "Galaxies are massive rotating systems of stars, gas, dust, and stellar remnants bound together by gravity. According to Newtonian mechanics, the orbital speed of stars should decrease with distance from the galactic center because the gravitational pull from visible matter weakens. However, observations of galactic rotation curves show that stars in the outer regions move just as fast as those near the center. Based on visible matter alone, these outer stars should