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Galaxy formation is the assembly of the matter and stellar populations that make a galaxy. In a standard dark matter framework, dark matter halos provide gravitational potential wells in which gas can collect. Radiative cooling lets gas lose thermal support and contract, while star formation converts some dense gas into stars.

The process is regulated rather than a one-way collapse. Stellar explosions, radiation and accreting central black holes return energy and momentum to the gas; galactic winds can remove some of it. Mergers and angular momentum transport change the geometry. A formation model must therefore explain both how material assembles and why only part of the available gas becomes a long-lived stellar population.

Joel Primack helped formulate cold dark matter as a quantitative framework for galaxy formation. The 1984 study with George R. Blumenthal, Sandra Faber and Martin Rees linked a primordial fluctuation spectrum to the formation of galaxies and galaxy clusters when collisionless particles have sufficiently small early velocity dispersion.^[ref]

Here “cold” concerns the particles' early random velocities, not whether the later galaxies are thermally cold.

Robert C. Kennicutt Jr. studies how gas becomes stars in galaxies. His 1998 observational analysis combined normal spiral galaxies and starburst galaxies, finding an approximately $1.4$ power law between disk-averaged star formation rate per unit area and gas surface density. This Kennicutt–Schmidt law connects measurements across whole galaxies to prescriptions used in models of galaxy formation.^[ref]

With Neal J. Evans II, he also reviewed how different observations estimate star formation rates and how the gas–star formation relation changes between molecular clouds and whole galaxies.^[ref]

Sandra Faber contributed to the cold dark matter explanation of galaxy formation. With George R. Blumenthal, Joel Primack and Martin Rees, she developed the 1984 account in which small early particle velocity dispersion preserves galaxy-scale density fluctuations against free streaming.^[ref]

The central test connects particle behaviour to astronomical structure: the surviving fluctuation spectrum determines which scales can seed galaxies and galaxy clusters.