Hundreds of millions of years after the Big Bang, charged, ‘ionised’ particles not seen since the earliest ages of the universe began to re-emerge. Named ‘reionisation’, this event was crucially important in the history of our universe – but because it occurred so far back in the past, telescope observations can only offer astronomers limited clues about how it unfolded. In his research, Dr Nick Gnedin at the Fermi National Accelerator Laboratory uses advanced computer simulations to study reionisation. His team’s project, named ‘Cosmic Reionization On Computers’, or CROC, now offers a key resource to researchers studying this distant period.