The evidence for the continuing accretion of gas by the Milky Way involves high-velocity clouds, or HVCs--mysterious clumps of hydrogen, moving rapidly through the outer regions of the galaxy.
Hydrogen can move through space as a cloud.
( z1 possible :ARG1 ( z2 move :ARG0 ( z3 hydrogen ) :ARG2 ( z4 space ) :prep-as ( z5 cloud ) ) )