Q: In your May feature, “Is the Big Bang in Crisis?” dark matter is called invisible. How was the dark matter in the story’s image of the El Gordo galaxy cluster detected and colorized if it is invisible?
William P. Gerhold
Hewitt, New Jersey
A: It is true that we cannot see dark matter directly. The blue color in the image of the El Gordo galaxy cluster (above) was artificially added to show where the cluster’s dark matter resides. Similarly, you’ve probably see