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Martin Rees develops theoretical accounts of cosmic structure. In a 1978 paper with Simon White, he separated the assembly of extended dark haloes from the dissipative concentration of luminous material within them.

Collisionless matter can build a large gravitationally bound halo, while gas can lose energy through radiative cooling and settle into a much denser luminous component. Their two-stage model connected the characteristic sizes and luminosities of galaxies to the cooling of gas inside a hierarchically assembled dark structure. It made the different physical behavior of the components central to the explanation.^[ref]

Simon White studies the formation of galaxies and cosmic structure. With Martin Rees, he proposed a two-stage account in which extended dark haloes assemble gravitationally and luminous galaxies form through cooling and concentration of gas within them.

The key distinction is dissipative: gas can radiate away energy, whereas collisionless halo material does not cool in the same way. The cooling requirement helps determine which dark structures can host concentrated luminous systems. Their 1978 model thereby connected the dark gravitational framework to the observed scale of galaxies.^[ref]