I don’t think that’s quite fair. That approach is exactly how we find planets. Here’s an unexpected variance in the motion of a planet or star. It could be explained by a planet over there. Oh look, there’s a planet over there.
Hypothesizing that a planet might be over there is a testable hypothesis.
Have we found a way to verify the presence of dark matter yet? Or is it still an untestable hypothesis sprinkled around distant galaxies so their acceleration curves look right?
That's not quite true. General relativity predicts gravitational lensing, not dark matter. Lensing has been used as an experimental probe for the presence of dark matter.
MOND is an alternative theory of gravity competing with GR. People usually forget that while MOND started to present a different explanation for Dark Matter, it is a theory of gravity. Dark Matter is not a theory of gravity and is compatible with GR.
Well of course. What I clearly meant was that DM predicts lensing effects in a magnitude that cannot be explained with ordinary matter. See bullet cluster or weak lensing observations.
I’m particularly amused by the hypothesis that spacetime can be bent without the presence of matter. We can’t detect dark matter because there’s no such thing, it’s just a brute topological fact.
Right, which is why it quickly led to the detection of dark matter...hmm.
I think a better analogy would be "that approach is exactly how we explain failing to find planets like Vulcan; we hypothesize that they are made of as-yet-unknown stuff that you can't see, touch, hear, smell, or in fact detect at all. But we know they're there because our calculations say they are."
It's actually a better example than you think. This exact theory led to long and protracted searches for the planet Vulcan, which would explain Mercury's strange behavior.
