After reading that exchange, it's pretty clear that Sean Carroll has basically ignored almost all the literature on modified gravity theories. It makes you wonder why he's commenting on it.
First, Bullet Cluster is not a problem for modified gravity. There have been numerous publications explaining why. In fact, it took much more effort to reconcile the Bullet Cluster with cdm than it did with modified gravity.
Secondly, clinging to the Bullet Cluster without mentioning Abell 520 is either ignorance or deception. Either way, there exist known celestial phenomenon (like galaxy rotation curves, the phenomenon that originally started the idea of dark matter) that are not adequately explained by dark matter theories (the same is true of modified gravity btw), and ignoring them in order to claim that we KNOW dark matter exists is intellectually lazy.
The problem with this discussion is that the intricacies of dark matter theories are much better understood by the cosmology community. Modified gravity on the other hand, it understood at best superficially by many cosmologists. As a result, whenever new findings in astronomy are published, there is a stampede of publications by people, who know dm but are ignorant of modified gravity, explaining why this "proves" once and for all why dark matter exists. The few modified gravity researchers out there are then left running around refuting all the false statements made about modified gravity.
I think Stacy hit the nail on the head with this quote:
I also agree that this [modified gravity theories] is contrived. But we are WAY into contrivance with LCDM(...). We’ve just gotten familiar with the contrived parts so that they no longer bother us. That doesn’t make them any less contrived.
While he doesn't always come off very well here, I think you're being too hard on him. He states outright in his earlier linked post[0] that he would prefer that modified gravity were true. He's at least familiar with TeVeS: I watched a video of a lecture he gave where he went into a fair bit of depth on it (i.e. way over my head). And he also seems to have done some small amount of modified gravity research himself (linked at the bottom of [0]).
My takeaway was that since the CMB spectrum seems to require dark matter, it's going to be there even if some form of modified gravity is also true. I've barely heard either side of the debate mention this in layman contexts. What's the modified gravity perspective on that?
The CMB spectrum does seem to require dark matter and it's a real problem for MOND. The point most MOND physicists keep trying to make is that dark matter has it's own share of problems but no one has discarded it as a theory yet.
First, Bullet Cluster is not a problem for modified gravity. There have been numerous publications explaining why. In fact, it took much more effort to reconcile the Bullet Cluster with cdm than it did with modified gravity.
Secondly, clinging to the Bullet Cluster without mentioning Abell 520 is either ignorance or deception. Either way, there exist known celestial phenomenon (like galaxy rotation curves, the phenomenon that originally started the idea of dark matter) that are not adequately explained by dark matter theories (the same is true of modified gravity btw), and ignoring them in order to claim that we KNOW dark matter exists is intellectually lazy.
The problem with this discussion is that the intricacies of dark matter theories are much better understood by the cosmology community. Modified gravity on the other hand, it understood at best superficially by many cosmologists. As a result, whenever new findings in astronomy are published, there is a stampede of publications by people, who know dm but are ignorant of modified gravity, explaining why this "proves" once and for all why dark matter exists. The few modified gravity researchers out there are then left running around refuting all the false statements made about modified gravity.
I think Stacy hit the nail on the head with this quote:
I also agree that this [modified gravity theories] is contrived. But we are WAY into contrivance with LCDM(...). We’ve just gotten familiar with the contrived parts so that they no longer bother us. That doesn’t make them any less contrived.