Imagine a slow moving blob heading towards a target on a wall, a small charge causes it to explode at some point along its path to the wall. The larger the charge the wider the splatter. In this case, the mass is the analogy to that detonation charge. If that blob was photons alone (which are the standard model assumption) you'd expect a smooth decreasing amount of splatter as you go away from the center of the target, what they seem to be seeing is an unexpected increase around 115 - 140 degrees.