It's been a very long time since I took meat science but in a very lay man's explanation it's this; rigor is the process of turning muscle into meat, the muscles have energy in them and it takes time for that energy to deplete and get the ph down, some butchers will actually hang red meat by the hip to increase the time it takes to use up that energy vs hanging by the tendons, dark cutters is a label of beef that is docked from the price and is caused by stress (it's argued whether taste is impacting in a noticeable way), rigor and cooling of meat is sorta hand in hand but there is a balance, cool too fast before muscle contract and it gets tough, butcher before rigor/cooling/contraction it gets tough.I'm not sure what difference that would make. Would like to hear more about the reason. Interesting
I'm sure I butchered some of that (pun sorta intended) but that's it in a nutshell. Aside from from special techniques (electrical shocking and immediate feinting of pork) red meat really needs to go through rigor to ensure you get the minimum tenderness required for adequate consumption. Hence, aging meat allows it to tenderize even more but at some point aging and tenderness stop and the only impact aging has is flavor profile.