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Not too long after the Big Bang, the cosmos was filled with an almost uniform soup of hydrogen and helium. Almost like wall-to-wall soup of super hot plasma of hydrogen and helium. And over billions of years, gravity condensed most of these gases into stars. Stars infused hydrogen and helium to form heavier elements like iron. And when large stars died in supernova explosions, naturally occurring elements heavier than iron were spewed out into the universe. Platinum, gold, things like that. These things compressed down under gravity to create small piles of rock. These then ended up like gravitation attracts to each other to form planets, asteroids, etc. So, outer space is nearly empty because most of the matter that was once out there is condensed into larger stuff like planets, moons and other things because of gravity. So, the expansion of space affects the distance between galaxies, but it doesn't increase the distance of stuff inside galaxies. Just talking.