> says that all other explanations besides baryonic are ruled out by data

That's something you don't see in physics. Ever.

There is always some alternative explanation that isn't ruled out by data. One may rule out all known explanations, but the explanation set is infinite and has a crazy topology. You can't ever exclude them all.

It is extremely challenging, perhaps impossible, to find a MOND theory that fits all available observations.

For me, a modification of gravity would be more aesthetically pleasing than dark matter, but that does not appear to be how Nature has chosen to be.

>It is extremely challenging, perhaps impossible, to find a MOND theory that fits all available observations.

All the commentators agree on this. What Ranier and Stacy are pointing out is that the same thing is true of LCDM. Dark matter does not do a good job of explaining galaxy rotation curves (even though this is the context which dark matter was originally proposed for), bullet cluster, abell 520, or lots of other galactic dynamics.

Of course, MOND has its own share of problems. Most importantly that Sean points out is the 3rd peak on the CMB. Stacy sounds like he's holding out for the possibility that relativistic formulations of MOND may explain this peak, though he admits that may not happen. To support his position, Stacy makes reference to other instances where MOND had unintuitive but correct conclusions (like for the bullet cluster).

usually the bullet cluster is cited as the exceptional observation for MOND, but that thing is a really strange beast and who knows what could be happening there: In fact one of the theories is that there's conventional lensing going on in the "area ahead of the bullet" due to plasma in the bow shock, which to me sounds way more reasonable than "a fleeing blob of dark matter that has been separated from the stars."