All the ‘stuff’ in the universe is made up of the same basic particles, arranged and connected in different ways.
We know about the elements of the period table. Well, each element exists as a different size atom. All atoms are combinations of three basic particles: protons, neutrons, and electrons.
If you drill down even further, we notice that protons and neutrons are made up of things called ‘quarks’. Electrons also belong in a family of similar particles called ‘leptons’.
Current theories are that there about 6 different quarks (which make things like protons and neutrons), and 6 different leptons – which come together in all sorts of combinations.
However, for the most part, these combinations are fixed as protons and neutrons don’t usually split apart. It’s much easier, and more relevant for everyday life, to think of all the stuff in the universe as made of elements like carbon, iron, oxygen and so on – as this is the level humans deal with.
Of course, a lot of the universe is just empty space – made of nothing at all!
As Aaron has said, the universe is made up of elements arranged in different ways to make a huge myriad of different types of chemical. These are spread out over vast distances. When the particles are large and close enough, they can attract each other through gravitational forces.
This video clip is a computer simulation made by my friends here at the University of Exeter, of the formation of star clusters that can occur when there’s enough ‘stuff’ close enough for this gravitational attraction to make cool things happen: https://www.youtube.com/watch?v=_c7rNfiQdjI