[mikeash]

If you're looking at the sky and there appears to be a lot more stuff there than just the bits that emit light, wouldn't Occam's Razor suggest that the presence of more stuff is the most natural explanation, rather than rejiggering the fundamental laws of physics?

[roryisok]

It's more like you're looking at the sky and what you can see doesn't weigh as much as it should. Does Occam's Razor suggest that there's invisible stuff? or that you're weighing stuff wrong?

[mikeash]

Except it's not, because the "missing" mass is not distributed in the same way as the stuff that emits light. Some places have a lot more "missing" than others.