NGC 1052-DF2 and DF4 are ultra-diffuse galaxies whose globular-cluster velocity dispersions of roughly 8 to 10 km/s are fully accounted for by their stellar mass alone: galaxies missing their dark matter. In ΛCDM this is doubly awkward. Galaxies are supposed to form inside dark matter halos, so a galaxy without one needs its halo removed after formation, and the standard mechanism, tidal stripping by the nearby giant NGC 1052, must be tuned to strip essentially all of the dark matter while leaving the diffuse stellar body and its anomalously luminous globular clusters intact. Two such galaxies in one group compounds the required luck, and the satellite trail proposed as the stripping signature ("bullet dwarf" collision, van Dokkum et al. 2022) remains debated.
The deeper problem is theoretical whiplash: dark-matter-deficient galaxies were first declared evidence FOR dark matter as a real, separable substance (since its presence elsewhere becomes meaningful only if it can be absent), yet they simultaneously stress the formation theory that requires halos as the sites of galaxy birth. The distance controversy (Trujillo et al. arguing 13 Mpc rather than 20, which would restore normal mass-to-light) has narrowed in favor of the larger distance via HST and JWST imaging, leaving the deficit standing. Meanwhile MOND struggles in the opposite direction: its prediction for an isolated galaxy of this mass overshoots the observed dispersions, requiring a delicate external-field rescue from NGC 1052's proximity.
The standing is an active anomaly with growing membership: additional dark-matter-poor candidates at various environments are accumulating, and the question has become structural: any theory of the dark sector must now explain both the rule (dark matter signatures everywhere) and the exceptions (specific galaxies with none), preferably with one mechanism rather than per-object tuning.