It's not theoretically undetectable, just very hard to detect, to the point that we haven't yet been able to do it. This article suggests that finally, we may be able to see it other than via gravitational effects.
For a long time I also subscribed to the idea that the missing matter was better explained by faulty math, somewhere, but as our observations get better it's looking more and more like there's really something there, that behaves independently and differently than the matter we know.
I just wonder if it isn't a side effect of some other (not understood) aspect of the universe rather than the result of the behaviour of some particle.
To illustrate, gravity per se doesn't exist, it's the outcome of the bending of space towards mass. So it's effect is real but it's not because some sort of gravitional particle is acting.
For a long time I also subscribed to the idea that the missing matter was better explained by faulty math, somewhere, but as our observations get better it's looking more and more like there's really something there, that behaves independently and differently than the matter we know.