EXTREME SCATTERING EVENTS AND GALACTIC DARK-MATTER

''Extreme scattering events'' (ESEs) are attributed to radio wave refr action by a cloud of free electrons crossing the line of sight. We pre sent 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 o ther models. The UV flux in the Galactic halo naturally generates elec tron densities of the right order We demonstrate, for the first time, a good reproduction of the prototypical ESE in the quasar 0954+658. Th e neutral clouds are a few astronomical units in radius and have masse s less than or similar to 10(-3) M.. The observed rate of ESEs implies that a large fraction of the mass of the Galaxy is in this form.