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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.

Read more at: http://phys.org/news/2014-04-hubble-team-monster-el-gordo.html#jCp

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?

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According to the original paper from the Atacama Telescope team the collision axis is somewhere between 15° and 30° to the line of sight. So the claim that the axis is tangential to the line of site is misleading (since the line of sight is a straight line, wouldn't the tangent to it be the same straight line?).

The velocity component normal to the line of sight is estimated at 586km/s (page 15 of the paper), so we'd expect to see some separation of the dark and bayonic matter distributions even though it wouldn't be as great as for the bullet cluster. For comparison, the collision speed in the bullet cluster is estimated to be 4500 km/s and the axis is roughly normal to the line of sight.

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  • $\begingroup$ ah, should have known better than not following to the source. In fact, sec. 4.1 mentions that there is no actual weak lensing map, so their dark matter map is just guesswork $\endgroup$
    – lurscher
    Commented Apr 4, 2014 at 17:54
  • $\begingroup$ The paper is from 2011. Didn't the dark matter distribution in the link you posted come from the more recent Hubble weak lensing study? $\endgroup$
    – John Rennie
    Commented Apr 4, 2014 at 18:07
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WMAP: 13.75 Gyr; 72% dark energy, 23% dark matter, 4.6% baryonic matter
Planck: 13.82 Gyr; 68.3% dark energy, 26.8% dark matter, 4.9% baryonic matter
http://arxiv.org/abs/1306.5534
Dark matter inside Saturn's orbit is observed to be less than ${1.7×10^{-10} M_{solar}}$.

One doubts "dark matter" materially exists versus Milgrom acceleration universally generating the Tully-Fisher relation, arXiv:1310.4009, 0906.0668 Milgrom acceleration is quantitatively sourced at ambient temperature within existing bench top apparatus over 90 days. The net output arises from chemistry not physics, and thus must be ridiculous. Keep dark matter postulated with continuing parameterizations; don't test the empirical alternative.

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    $\begingroup$ conformal Weyl gravity and some other variants do MONDesque predictions like Tully-Fisher, but what they don't explain is the Bullet Cluster weak lensing pattern $\endgroup$
    – lurscher
    Commented Apr 4, 2014 at 17:56
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    $\begingroup$ @lurscher: There is some work being done on MOND predictions of the Bullet Cluster (e.g., this one) $\endgroup$
    – Kyle Kanos
    Commented Apr 4, 2014 at 20:34
  • $\begingroup$ Thank you! Dark matter parameterizes everything except its own local empirical non-existence. MoND has no adjustments, but Milgrom acceleration must be sourced. Physics already has the universal source, but won't test for it. That is cowardice. Cartography shows Euclid is incomplete. This is not disaster, this is Bolyai (then Thurston). $\endgroup$
    – Uncle Al
    Commented Apr 4, 2014 at 22:12

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