But when we look at galaxies and so on, things don't seem to add up if you only look at the visible matter.
So, there could be a lot of dark matter we can't see. Or GR might not be correct and MOND (or a relativistic formulation like AQUAL) might be the right answer.
It's true I guess that if GR is correct, then we need some kind of dark matter to explain observations. But it might not be correct.
But when we look at galaxies and so on, things don't seem to add up if you only look at the visible matter.
So, there could be a lot of dark matter we can't see. Or GR might not be correct and MOND (or a relativistic formulation like AQUAL) might be the right answer.
It's true I guess that if GR is correct, then we need some kind of dark matter to explain observations. But it might not be correct.