In a string of a dozen small galaxies hung in the sky like a row of cosmic fairy lights, astronomers have discovered a small, ghostly galaxy without any dark matter. The rare discovery offers additional evidence that violent galactic collisions can knock this invisible substance loose from visible matter.
Dark matter acts like cosmic glue, accumulating in "halos" in which galaxies form. It makes up about 85% of all the matter in the cosmos, meaning all the stars and other celestial objects we see account for a small fraction of the universe's mass. And as astronomer Vera Rubin observed, despite its invisible nature, dark matter seems essential to explain the apparent movement of stars around their galactic centers.
