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.
We know everything there is to know about masses of hydrogen.
( z1 know :ARG0 ( z2 we ) :ARG1 ( z3 everything :ARG1-of ( z4 know ) :topic ( z5 mass :cxnsistof ( z6 hydrogen ) ) ) )