By looking at how the faint light from galaxies behind the cluster they were studying was distorted by that cluster, Dr Jee and his team created a map of the distribution of its mass. They then compared that with what they could actually see. Instead of finding that the mass coincided with the location of the ordinary matter of stars, as had been seen in observations of other clusters, they found a distortion.
After trying—and failing—for many months to explain this distortion away, they accepted that it was real, and sought to explain it. The most plausible explanation is that the cluster contains a distinct ring of dark matter without any accompanying ordinary matter.
While this may not identify exactly what dark matter is, if confirmed, then it means that both Newtonian and Einstein's gravitational laws are accurate without any need for modification, at least not based on this.
We must still wait for the paper's publication and the scrutiny of people in this field.
Edit: This is the NASA Press release related to this.