In article , Steve Willner
writes:

One fly in the lensing ointment is
that measurements have been made only towards the LMC and the
Galactic center (unless I've missed other observations, which is
quite possible). If you can arrange for red dwarfs to occupy regions
other than these lines of sight and still explain the rotation
curves, you might get somewhere.

I'm pretty sure that there were observations of M31 looking for
microlensing effects.

For larger masses, one can rule them out because they would be visible
in QSO light curves. (Hawkins claims such a detection of dark matter,
but it doesn't stand up to a quantitative analysis.)