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Many of the satellites of galactic-mass systems such as the Miky Way,Andromeda and Centaurus A show evidence of coherent motions to a larger extentthan most of the systems predicted by the standard cosmological model. It is anopen question if correlations in satellite orbits are present in systems ofdifferent masses. Here , we report an analysis of the kinematics of satellitegalaxies around massive galaxy groups. Unlike what is seen in Milky Wayanalogues, we find an excess of diametrically opposed pairs of satellites thathave line-of-sight velocity offsets from the central galaxy of the same sign.This corresponds to a $\pmb{6.0\sigma}$ ($\pmb{p}$-value $\pmb{=\9.9\times10^{-10}}$) detection of non-random satellite motions. Such excess ispredicted by up-to-date cosmological simulations but the magnitude of theeffect is considerably lower than in observations. The observational data isdiscrepant at the $\pmb{4.1\sigma}$ and $\pmb{3.6\sigma}$ level with theexpectations of the Millennium and the Illustris TNG300 cosmologicalsimulations, potentially indicating that massive galaxy groups assembled laterin the real Universe. The detection of velocity correlations of satellitegalaxies and tension with theoretical predictions is robust against changes insample selection. Using the largest sample to date, our findings demonstratethat the motions of satellite galaxies represent a challenge to the currentcosmological model.

A younger Universe implied by satellite pair correlations from SDSS observations of massive galaxy groups