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.
There is a type of gas moving through the galaxy.
There could be a gas, but it is also mysterious so it could not be a gas in the end.
( z1 move :ARG1 ( z2 gas :mod ( z3 type ) ) :ARG2 ( z4 galaxy ) )