Ethan Siegel in Forbes:
When you look out at the Universe, all you see is matter and light. The stars, galaxies, plasmas, and unusual astrophysical objects all emit radiation from across the electromagnetic spectrum; the dust, gas, and neutral atoms absorb it. Yet what we infer from viewing them, particularly on the largest scales, tells us that there's much more than what we presently perceive. In addition to matter and light, there's got to be dark energy, a form of energy inherent to the fabric of space itself that causes the expanding Universe to accelerate, and a significant amount of dark matter: massive, clustering particles that are invisible to light. Dark matter can do many things, but one prediction it's always struggled with is exactly reproducing how galaxies are observed to rotate. It's been a problem for decades, from the 1970s until 2017. But as of June 23rd, a new paper claims to have finally solved the problem of galactic rotation at long last.
Since 1970, it's been known that galaxies don't just rotate, but they rotate with speeds too quick, particularly at the outskirts, for what normal matter alone can account for. Nearly half a century of studies have shown that if dark matter exists, it should form diffuse, massive halos that extend much farther than the visible disks and elliptical swarms do, with the gravity of both dark and normal matter affecting the galaxies' motion.