The effect of microgravity on the growth of silica nanoparticles via the sol-gel route is profound. In four different recipes that typically produce silica nanoparticles in unit gravity, low-density gel structures were instead formed in microgravity. These observations suggest that microgravity reduces the particle growth rate, allowing unincorporated species to form aggregates and ultimately gel. Hence microgravity favors the formation of more rarefied structures, providing a bias toward diffusion-limited aggregation. Moreover, these results add to evidence that the growth of silica nanoparticles occurs not simply through monomer addition but by the attachment of smaller primary particles and aggregates.