ACCORDING to the favoured models for the formation of large-scale structure in the Universe (in which the dynamics of the Universe is dominated by cold dark matter), the distribution of galaxies and clusters of galaxies should be random on large scales. It therefore came as a surprise when a periodicity was reported1 in the distribution of high-density regions of galaxies in the directions of the Galactic poles, although the apparent lack of periodicity in other directions led to the initial report being regarded as a statistical anomaly2. A subsequent study3-6 also claimed evidence for periodicity on the same scale, but the statistical significance of this result was uncertain due to the small number of clusters used. Here, using a new compilation7 of available data on galaxy clusters, we present evidence for a quasi-regular three-dimensional network of rich superclusters and voids, with the regions of high density separated by ~120 Mpc. If this reflects the distribution of all matter (luminous and dark), then there must exist some hitherto unknown process that produces regular structure on large scales.