What is the universe made of? What is ‘dark matter’? Why is the universe still expanding? These are just some of the questions astrophysicist Professor Katherine Freese tackles in this wonderfully accessible interview. The ordinary atoms that make up the known universe—from our bodies and the air we breathe to the planets and stars—constitute only 5 percent of all matter and energy in the cosmos. The rest is known as dark matter and dark energy, because their precise identities are unknown. Prof Freese explains that new galaxies come about when dark matter clumps together, indeed it is dark matter that dominates structure formation rather than atoms. However, we are not sure what dark matter actually is. Prof Freese is one of the world’s leading astrophysicist, but this is an interview that the layperson can enjoy.