Both / they're related. More mass gravitates more. Suppose you had an infinite universe that was filled with a cool gas of low uniform density. Then the grav

Both / they're related. More mass gravitates more. Suppose you had an infinite universe that was filled with a cool gas of low uniform density. Then the gravitational field at any particular point would be 0, by Gauss' law. But, if you wait a brief moment the gas will not stay uniform, because each atom of the gas will have some velocity. You'll observe fluctuations: places with small over-density and small under-density (compared to the average). The places with over-density will gravitate more than the average and places with under-density will gravitate less, and gravity will cause the gas to clump. Wait a few billion years and some places will have amalgamated whole galaxies' worth of matter around them and other places will be empty.