The key finding of this paper was instantly apparent to anyone who compared these two maps: whereas the bulk of the visible matter in this system—the hot gas—was confined largely to the innermost volume of these clusters, the center of gravity was not. For the first time, astronomers had found a system in which the source of the gravity—and thus dark matter—was not centered around the same place as the visible matter. No MOND theory predicted anything like this. Knowing this, the authors of this paper gave it the title “A Direct Empirical Proof of the Existence of Dark Matter.”
