Most of the matter in the universe, according to the standard cosmological model, is dark—invisible stuff that interacts gravitationally but as far as we know is not subject to the electromagnetic or any other interaction. Gravity is enough, however, to ensure that a spherical halo of dark matter surrounds a galaxy’s shining stars. Now a team of astrophysicists led by Durham University’s Richard Massey has spotted a galaxy in which the halo appears to be trailing the luminous matter as both are attracted toward the cluster center. Massey and colleagues suggest that the offset may result from frictional interactions between the halo and the dark matter of the cluster core. The galaxy, labeled N1 in the image (taken with the Hubble Space Telescope), is one of four galaxies (N1–N4) in the core of the Abell 3827 cluster. The misalignment of N1 with its halo followed from the team’s detailed analysis of the severely distorted, gravitationally lensed distant galaxy visible in two pieces: an arc partially surrounding the four galaxies and the structure near N1. The offset of dark and luminous matter might arise from dark-matter self-interactions. But the possibility remains that combined effects of matter along the line of sight to Abell 3827 and conventional physics in the complex environment of the cluster could somehow be responsible for the misalignment. Detailed simulations and improved modeling of dark-matter interactions should help clarify whether the offset really manifests unconventional physics. (R. Massey et al., Mon. Not. R. Astron. Soc. 449, 3393, 2015.)