In both experiments, what accounts for the change in pressure you observed as you changed the volume of the gas

When a gas is in a smaller volume, the molecules are more confined and bump into the walls of the flask more, thereby exerting a greater pressure.

Explanation:

i guess that visual aids present persuasive examples to what you are presenting, some factors would be theme (does it fit with the color, information, what you are presenting), size (can you see it, is it a big enough span of data/information), and overall look of the visual aid.

hope this : )

3. dark matter

empty space could also be a simple vacuum