  Hierarchical clustering models of cold dark matter (CDM) predict that about
5% - 10% of a galaxy-sized halo with mass ~ 10^12 solar masses (M_sun) resides
in substructures (CDM subhalos) with masses <= 10^8 M_sun. To directly identify
such substructures, we propose to observe radio continuum emission from
multiply imaged QSOs using VSOP-2 with a high angular resolution.
