Question: Do galaxies do anything to space, or do they just fill it up?

Sam Geen answered on 17 Jun 2013:

The matter in galaxies does change the expansion of space – having more matter in the universe changes its overall density, which changes how the universe expands. They also bend space, which can cause light to be bent too as it travels through space – you can sometimes see weird lens effects where light from behind a galaxy gets bent into banana shapes.