Ethan Siegel in Forbes:
One of the greatest puzzles in the entire Universe is the dark matter mystery. In theory, for every bit of normal matter (like us) in the Universe, there should be approximately five times as much dark matter. Both normal and dark matter should experience gravitation equally, meaning that the largest structures in the Universe — galaxies, clusters of galaxies, and cosmic filaments — should contain and be dominated by dark matter. When we measure the motions of individual galaxies, both isolated and in clusters, the normal matter alone is not enough to explain what we see. Dark matter is also required.
But the Universe is also a violent place, full of mergers, collisions, and cosmic smash-ups. Some of these events should expel enough normal matter to create new, small, dark-matter-free galaxies: with normal matter alone. For the first time, scientists believe they’ve found one such galaxy with no dark matter, solving an enormous cosmic puzzle.