Abstract

The standard cold dark matter plus cosmological constant model predicts that galaxies form within dark-matter haloes, and that low-mass galaxies are more dark-matter dominated than massive ones. The unexpected discovery of two low-mass galaxies lacking dark matter immediately provoked concerns about the standard cosmology and ignited explorations of alternatives, including self-interacting dark matter and modified gravity. Apprehension grew after several cosmological simulations using the conventional model failed to form adequate numerical analogues with comparable internal characteristics (stellar masses, sizes, velocity dispersions and Sérsic indices). Here we show that the standard paradigm naturally produces galaxies lacking dark matter with internal characteristics in agreement with observations. Using a state-of-the-art cosmological simulation and a meticulous galaxy-identification technique, we find that extreme close encounters with massive neighbours can be responsible for this. Moreover, these encounters occur when the satellite is still gas-rich, suggesting that gas-rich interactions may explain the peculiar globular cluster populations in these galaxies. We predict that ∼30% of massive central galaxies (with at least 10e11 solar masses in stars) harbour at least one dark-matter-deficient satellite (with 10e8 -10e9 solar masses in stars). This distinctive class of galaxies provides an additional layer in understanding how tidal stripping and star formation triggering unfolds under these extreme conditions. Future observations surveying galaxies in the aforementioned regime will provide a crucial test of this scenario. If time allows, I will also mention recent results on the radial acceleration relation and challenges to MOND.