system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is space a vacuum?
gpt
Because there's far, far more space than there is stuff to fill it. This is due to the fundamental forces of our universe (mostly gravity) which tend to make stuff attracted to stuff. That's why you see small (astronomically speaking) clumps of matter and energy (like a galaxy) surrounded by a vast, vast sea of near-nothingness. 

(Also, space isn't a _true_ vacuum. There's _some_ matter and energy floating around in even the most remote swaths of the void.)