  Observations of high redshift type Ia supernovae (SNe) will enable us to
probe the structure of galaxy halos and the composition of dark matter. The
future prospects for this field are briefly discussed here. First the ability
of SN observations to differentiate between dark matter made of macroscopic
compact objects and dark matter made of microscopic particles is reviewed. Then
a new method for probing the structure of galaxy halos and galaxy cluster halos
is described. This method utilizes the correlations between foreground galaxy
light and supernova brightnesses to substantially decrease possible systematic
errors. The technique may be particularly useful for measuring the size of dark
matter halos, a measurement to which the galaxy--galaxy lensing is not well
suited, and the level of substructure in galaxy halos, a problematic prediction
of the cold dark matter model. The required observations of hundreds of SNe at
z ~ 1 are already being proposed for the purposes of cosmological parameter
estimation.
