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.

So how does one weigh a cluster of galaxies almost 10 billion light years away and 3 million billion times more massive than our sun? Very carefully, it turns out.

We've previously covered gravitational lensing, a phenomenon that occurs around clusters of galaxies where the combined gravity is so strong that it warps space and light in a way that actually magnifies our view of space far beyond what we'd be able to see otherwise.

Weak lensing works on a similar principle — except instead of forming a zoom lens, the reflection it returns is closer to that of a funhouse mirror.

By measuring just how much the cluster's gravity had distorted the pictures of space around it, James Jee from the University of California Davis was able to calculate that we'd vastly underestimated the cluster's mass by about 43%.


Image: NASA, ESA, and J. Jee (University of California, Davis)