The behavior of distant galaxies actually doesn’t match physics on earth. It’s a huge open problem. The leading hypothesis is dark matter, but that resists experimental confirmation despite extended effort, and has plenty of doubters.

Besides that, I think we’re assuming too much about their motives and capabilities. Who’s to say simulating dust clouds isn’t the point, and we’re just an accident? Or that the point is a survey of the evolution of astronomical systems, or an attempt to predict the fate of the universe? And, since any technology that could simulate even this planet is completely beyond human comprehension, why should we expect them to scale up only about as efficiently as humans do, rather than, say, orchestrating some reaction that results in a computer that grows exponentially and boundlessly? Or that the purpose or mechanism are as beyond our comprehension as a machine that could simulate Earth? Or, if they do have a point of limiting cost, why should we have any idea where that is, and where the cost is so low that resources are spent like they’re free?