Massive clusters of galaxies have been found that date from as early as 3.9 billion years¹ (3.9 Gyr; z = 1.62) after the Big Bang, containing stars that formed at even earlier epochs^2,3. Cosmological simulations using the current cold dark matter model predict that these systems should descend from ‘protoclusters’—early overdensities of massive galaxies that merge hierarchically to form a cluster^4,5. These protocluster regions themselves are built up hierarchically and so are expected to contain extremely massive galaxies that can be observed as luminous quasars and starbursts^4,5,6. Observational evidence for this picture, however, is sparse because high-redshift protoclusters are rare and difficult to observe^6,7. Here we report a protocluster region that dates from 1 Gyr (z = 5.3) after the Big Bang. This cluster of massive galaxies extends over more than 13 megaparsecs and contains a luminous quasar as well as a system rich in molecular gas⁸. These massive galaxies place a lower limit of more than 4 × 10¹¹ solar masses of dark and luminous matter in this region, consistent with that expected from cosmological simulations for the earliest galaxy clusters^4,5,7.

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