translation.en: And, as we've learned in the last few years, galaxies are held together by the gravitational pull of so-called dark matter: particles in huge swarms, far smaller even than atomic nuclei.
translation.zh: 我们近年来已了解到星系也是由 所谓的暗物质的引力作用连系在一起。 暗物质粒子比原子核要小很多，且大量密集。