I very much agree with you, and have raised these questions before. I got some interesting responses that half-mollified my worries. Originally dark matter was

I very much agree with you, and have raised these questions before. I got some interesting responses that half-mollified my worries. Originally dark matter was hypothesized because of galaxy rotation mismatches. This allows for mapping where 'dark matter' should be. One falsification of dark matter that was tried is gravitational lensing. If there is indeed extra curvature of space-time in these galaxies, then they should bend light-rays, Potentially showing the same galaxy twice. This was tested, and confirmed. Hypothesized dark matter, as detected by galaxy rotation, causes the expected gravitational lensing. I suppose another potential falsification is by 'moving' dark matter through a gravitational tractor (very heavy mass with rocket engines) and then detect the resultant change in gravity. You could attempt that detection by gravitational lensing, orbit velocity, or perhaps something else. Despite all this, dark matter still feels like our theory of gravity was falsified, and instead of concluding the theory was wrong, it was attempted to add random parameters to the theory. Its not like there is a better theory though (I believe MONS has trouble with explaining gravitational lensing), so perhaps this is a problem of PR much more than a problem of science. As for the second question, for dark matter to be matter, I think it suffices if it has mass and inertia. My gravity tractor detection idea tries to work with that. Edit: also see this: https://medium.com/starts-with-a-bang/7-independent-pieces-of-evidence-for-dark-matter-3692126a2283 https://medium.com/starts-with-a-bang/7-independent-pieces-o... from a different reply in this HN discussion.