Sorry, I wasn't specific enough in my explanation. GP was referring to micro-lensing experiments, i.e. lensing around a star or other such object. That experiment is essentially selecting a bright point light source behind a galaxy, and looking for the sort of distortion that would occur if there was an object near the light's path as it passed through the galaxy. If you see lensing but no object, you've found a dark, dense object.

https://en.wikipedia.org/wiki/Gravitational_microlensing

You're right that all the dark matter in a galaxy would contribute to gravitational lensing around that galaxy, but that's a different signal; you'd be looking for the lensing at different scale, e.g. Einstein Rings around the edge of the galaxy, not inside it.

I meant 'diffuse' literally, in the physical sense, as in sparse, not dense -- the 'missing mass' that we're trying to explain is, under the dark matter hypothesis, spread out more thinly than if it was accounted for by dense dark objects like planets or stars.