We already knew that this ancient galaxy cluster (nicknamed "El Gordo") was the largest of the distant galaxies that we had spotted, but it turns out we've been massively underestimating its size. How did astronomers figure it out? Simple. They weighed it.
This galaxy cluster is nicknamed El Gordo, which means "The Fat One" in Spanish. Located seven billion light-years away, it's the biggest cluster ever found at that distance — and it's resounding proof that dark matter is everywhere in the cosmos.