This region, called the galaxy halo, is mostly made up of invisible dark matter acting as the galaxy’s gravitational glue. The rest of the halo consists of normal matter, including stars and hot, charged gas. Arrakihs will observe diffuse stellar haloes and structures such as stellar streams – remnants of small galaxies that were torn apart by gravity.

Importantly, a galaxy’s halo contains clear traces of how the galaxy formed and evolved over cosmic time. Scientists believe that galaxies grow over time by merging with others. Because galaxy haloes are so faint, we haven’t been able to study enough of them to be sure that our models of galaxy formation – and by extension the role of dark matter – are correct.

By mapping stellar streams, Arrakihs will allow us to piece together the history of past mergers and give an estimate of the number of ‘lonely’ stars that were ripped away from their galaxies during mergers.

In total, Arrakihs plans to investigate at least 80 galaxies with a similar mass to the Milky Way galaxy. This is a large enough number to get statistics on how a ‘typical’ galaxy forms, allowing us to understand how unique our home galaxy is.