What is space made of? If there is an estimated one atom per cubic meter of space, what else is filling up space?
Space is primarily a vacuum, as far as we know.
This can be a difficult concept for some, but most of space contains no matter whatsoever–it's just emptiness. Dark Matter, a little understood thing that seems to have gravity but does not interact with electromagnetic radiation, may fill of some (or maybe a lot) of this space, but scientists are VERY uncertain, As of right now, space is considered to be a vacuum except for the small amount of normal matter in it.