  Studies of galaxy surveys in the context of the cold dark matter paradigm
have shown that the mass of the dark matter halo and the total stellar mass are
coupled through a function that varies smoothly with mass. Their average ratio
M_{halo}/M_{stars} has a minimum of about 30 for galaxies with stellar masses
near that of the Milky Way (approximately 5x10^{10} solar masses) and increases
both towards lower masses and towards higher masses. The scatter in this
relation is not well known; it is generally thought to be less than a factor of
two for massive galaxies but much larger for dwarf galaxies. Here we report the
radial velocities of ten luminous globular-cluster-like objects in the
ultra-diffuse galaxy NGC1052-DF2, which has a stellar mass of approximately
2x10^8 solar masses. We infer that its velocity dispersion is less than 10.5
kilometers per second with 90 per cent confidence, and we determine from this
that its total mass within a radius of 7.6 kiloparsecs is less than 3.4x10^8
solar masses. This implies that the ratio M_{halo}/M_{stars} is of order unity
(and consistent with zero), a factor of at least 400 lower than expected.
NGC1052-DF2 demonstrates that dark matter is not always coupled with baryonic
matter on galactic scales.
