It is mostly space
Teaching Guidance for 5-11
On average, there's not much in the universe
Wrong Track: The universe is rather full of solid objects, much like around here.
Right Lines: The universe is mostly empty space, containing large, well-separated objects.
A few molecules in each cubic metre
Thinking about the learning
Children tend to have a very crowded view of space and this is not helped by the various diagrams of the Solar System that can be found on the internet. In order to see all the planets in one diagram, it has to be grossly out of scale.
Thinking about the teaching
You'll want to do some explicit modelling of the scales and distances to counter the distortions of the diagrams, and our everyday expectations which suggest that (local) space is rather full of things.