I have a #physics question and I hope maybe some nerd will be interested in taking up the challenge of explaining something to an (I like to think) intelligent layperson. A lot of stuff I read about dark matter seems to assume, implicitly or even explicitly, that it comprises particles that not only don't interact with the ones we're pretty sure about, but also don't interact with each other. Is this just Occam's Razor? 'Cause, I mean, everything we DO know about is *complicated*! …