Hey Siri: How much does this galaxy cluster weigh?

It’s been nearly a century since astronomer Fritz Zwicky first calculated the mass of the Coma Cluster, a dense collection of almost 1,000 galaxies located in the nearby universe. But estimating the mass of something so huge and dense, not to mention 320 million light-years away, has its share of problems—then and now. Zwicky’s initial measurements, and the many made since, are plagued by sources of error that bias the mass higher or lower.