According to this news,
The expectation of "unaccounted energy" comes from the fact the merger of galaxy clusters is occurring tangentially to the observers' line-of-sight. This means they are potentially missing a good fraction of the kinetic energy of the merger because their spectroscopic measurements only track the radial speeds of the galaxies.
What strikes as a surprising is that the picture shows the dark matter distribution (inferred from weak lensing) in blue hue, and it shows a similar pattern that Bullet Cluster, even though in the Bullet Cluster case, the collision happen perpendicular to the line-of-sight
Any idea why so much discrepancy between normal matter and dark matter distribution along that axis?