I was born on 30 September 1951 to Gwynneth Hallett, a housekeeper in a small seaside boarding house in Kent. I never met my father. My mother and I moved from one live-in job to another until 1956 when she married her then employer, George Symmons, a retired naval cook 30 years her senior. That year, I entered a one-room rural primary school, walking two miles each way from our cottage in South-East Cornwall. Struck by my precociousness, the teacher alerted the county educational psychologist and a few years later I took the entrance exams for Christ’s Hospital, a school for poor children founded in 1552 in the City of London but now located in the Sussex countryside.

I attended CH (then a boarding school for boys only) from 1962 until 1968. A reserved schoolboy, my academic strengths were in languages, mathematics and physical sciences, I enjoyed choral and instrumental music-making and I was an indifferent but enthusiastic sportsman. In late 1968 I won an Exhibition to study mathematics at Jesus College, Cambridge. Between school and university I spent six months in Paris following courses in French civilization at the Sorbonne. This created a lifelong interest in French language, literature, art and culture.

In Cambridge, I found myself gradually more attracted to Applied than to Pure Mathematics. I also took up rowing and spent part of my summers on student-run holiday camps for disadvantaged German children, resulting in greatly improved spoken German which was to be useful later. In my final year I was particularly enthused by a stellar dynamics course from Donald Lynden-Bell, so when Donald offered me a PhD place at the Institute of Astronomy, I gladly accepted.

First, however, I spent a year in Toronto, learning some observational astronomy and spending three weeks in Chile on a project which led to my first publication, a photometric study of a young star cluster. During this year, I became interested in the mystery of the “missing mass” and formulated the main project for my PhD, an exploration of whether the dark matter in galaxy clusters could be bound to the individual galaxies. I addressed this with my first computer experiment, a simulation of cluster formation where all mass was attached to galaxies. Finishing my thesis early, I spent my last few student months simulating galaxy mergers and working with Martin Rees on what has become the standard paradigm for galaxy formation, the cooling and condensation of gas within massive halos that grow gravitationally out of pre-existing dark matter.

Over the next six years, I had postdocs in Berkeley and Cambridge and made extended visits to the National Radioastronomy Observatory, the Institute for Advanced Study, the Institut d’Astrophysique de Paris and the Institute for Theoretical Physics. During this period I met Carlos Frenk, who became a lifelong collaborator, and Judith Jennings, a Californian engineer with whom I shared an enthusiasm for English Folk Dance. We got married in 1984. In Berkeley, Carlos and I worked with Marc Davis to show that dark matter could not be made of massive neutrinos, and then joined with George Efstathiou in a simulation project which showed that Cold Dark Matter (CDM), a new kind of elementary particle, could explain the observed large-scale structure of the Universe.

Although the CDM hypothesis was viewed with scepticism in the 1980’s, it had enough impact that I was offered a tenured faculty post at the University of Arizona in 1984, and within a few years I became Full Professor in the Steward Observatory. Theoretical astrophysics grew substantially during my time in Arizona, but by the end of the decade I had become unhappy with my personal and social situation and in 1990 I moved back to Europe to a post at the Institute of Astronomy in Cambridge.

At IoA, I worked with Carlos to put my early galaxy formation ideas into a CDM context. This was extended by my student and second wife Guinevere Kauffmann to create “semi-analytic modelling”, a technique which later allowed us to populate cosmological simulations with galaxies, culminating in the well-known Millennium Simulation of 2005. In 1994 I became director of the Max Planck Institute for Astrophysics (MPA) in Garching, and moved there with Guinevere who subsequently provided key observational constraints for semi-analytic models using large galaxy surveys, and was appointed scientific director at MPA in 2013. Our son Jonathan was born in 1996. In this period, I also worked with Carlos Frenk and Julio Navarro on my most highly cited work, showing that dark matter halos have a simple structure which is closely related to the properties of the universe in which they form.

For two decades my MPA group and I have worked with collaborators worldwide to produce ever more realistic simulations of cosmic structure formation. This programme would have been impossible without my former MPA student, Volker Springel who created much of the necessary software infrastructure and carried out some of the highest impact simulations, in particular, the Millennium Simulation. In 2017 Volker was offered a directorship at MPA as my successor. For 40 years I have been lucky to ride a tidal wave of progress both in numerical simulation technology and in our understanding of cosmic structure formation. This has been extraordinarily exciting, and I am grateful to the Max Planck Society for providing an ideal context for the second half of this career. On June 24 2016, the day after the Brexit vote, I filed papers for German citizenship.

26 September 2017 Hong Kong