Question about the universe in relation to galaxies and space?

Scientists say that our universe is expanding because the galaxies keep moving away from each other. If the galaxies are moving away from each other what is the a space in between the galaxies. For example if the 0 represents a galaxy, these are two galaxies next to each other 00. But when they start to move away and the universe expands it would look like this 0 0. What is the space in the middle 0 HERE 0. What is this space called? And would there be any planets, stars, asteroids?