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.
Clumps of hydrogen can not move through the vacuum of space.
( z1 possible :polarity - :ARG1 ( z2 move :ARG1 ( z3 cluster :cxnsistof ( z4 hydrogen ) ) :path ( z5 vacuum :mod ( z6 space ) ) ) )