> Immediately I see this as being a gravitational issue. Maybe gravity works in ways we do not understand at this level. But instead scientists jump directly to

> Immediately I see this as being a gravitational issue. Maybe gravity works in ways we do not understand at this level. But instead scientists jump directly to "there must be matter here that we can't see". You go for the most likely explanation first. When I go to my car and my keys aren't in my pocket, I go and check the counter where I usually put them when I get home. I don't start checking behind the toilet or similar. In this case we had a theory, general relativity, which worked very well for all the tests we'd done so far, and there was nothing in the theory to indicate it shouldn't be the same all over. Meanwhile astronomical observations are difficult and we knew, and still know, there's more to particle physics. Hence more mass is a more likely explanation. Keep in mind that trying to replace an existing, validated theory can be very hard, since you need to not just explain the new stuff, but also pass all the checks the existing theory did. So far nobody has been able to come up with a modification of general relativity that can do this. And it's not been for lack of trying[1]. [1]: https://en.wikipedia.org/wiki/Alternatives_to_general_relativity https://en.wikipedia.org/wiki/Alternatives_to_general_relati...