  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 morphologies). 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. We predict that approximately 30 percent of massive central galaxies
(with at least 1e11 solar masses in stars) harbour at least one
dark-matter-deficient satellite (with 1e8 - 1e9 solar masses in stars). This
distinctive class of galaxies provides an additional layer in our understanding
of the role of interactions in shaping galactic properties. Future observations
surveying galaxies in the aforementioned regime will provide a crucial test of
this scenario.
