A distant supercluster of galaxies points to self-interacting dark matter

Seven billion light-years from Earth, there is an unexpected cluster of galaxies. It has two regions in the process of colliding with each other with a combined mass of more than two quadrillion solar masses. Given its mass, the distant cluster has been nicknamed El Gordo, the Fat One. What makes it unusual is that it's extraordinarily large for that period of the universe. How did such a large supercluster form so early?