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>Why not just say: "We don't understand the large scale effects/properties of matter."
Because that's not a scientific hypothesis. This statement does not make any predictions of future observations. Throwing your hands up in defeat doesn't offer any way to study the problem, it's pointless. Quantum mechanics isn't just "weird shit happens, idk", there are quantitive models which describe the behaviour of such systems. In QM you cannot calculate the outcome of a single experiment, but you can predict the average result and the distribution. So it's predictive. Models need to be specific so they can be tested. There are infinite ways that physics could be altered on large scales, obviously that's not falsifiable.
>Rather than "A new form of matter exists."
That's not how it progressed historically. Originally there were was no reason to believe this matter was special, it could just be low-luminosity matter like brown dwarfs, or cold gas. It was only after a few decades that the case was built that dark matter wasn't normal matter. And at this point people also pursued alternative models, where either gravity was altered or you have additional forces. Those alternatives never got very far, critically they failed to predict the earliest observations of large scale structure in the Cosmic Microwave Background. The Cold Dark Matter model got the prediction right, decades later alternative models still haven't got over this. The most famous alternative (MOND) actually requires dark matter of it's own, and it still has issues.
This is the reason dark matter is the standard explanation, it's had many predictive successes across a huge range in scales. Alternatives on the other hand are a long way behind.
