But they don't. We know they don't. Not unless you use a weird definition of orbit that is very different from the one lotsofpulp was using. And if you do that you're not countering their argument, you're misconstruing it.
And our universe has tons of matter with gravitational mass everywhere, few other types of interaction beyond gravity, and a vacuum that just doesn't want to stay empty.
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[0] - Not sure if this was mathematically proven, or merely remains not disproven.
Actual orbits being slightly off ellipses isn't what I meant.
All of science is like this. Change your frame of reference/theory. Why did we pick one system vs another? Its arbitrary.