Extreme scattering events and Galactic dark matter

"Extreme scattering events" (ESEs) are attributed to radio wave refraction by a cloud of free electrons crossing the line of sight. We present a new model in which these electrons form the photoionized "skin" of an underlying cool, self-gravitating cloud in the Galactic halo. In this way, we avoid the severe overpressure problem that afflicts other models. The UV flux in the Galactic halo naturally generates electron densities of the right order. We demonstrate, for the first time, a good reproduction of the prototypical ESE in the quasar 0954+658. The neutral clouds are a few astronomical units in radius and have masses ≲10^-3 M_⊙. The observed rate of ESEs implies that a large fraction of the mass of the Galaxy is in this form.