The universe is made up of all of the items that can be a member of a set.
Every item is a member of exactly one set.
For example, suppose the items in our universe are corporations that still exist today or were acquired by other corporations. Our sets are corporations that still exist under their own name. We will limit ourselves to two operations. Suppose we take the union of Piedmont Air and Empire Air and called the resulting corporation Piedmont Air. Suppose we further unite US Air with Piedmont Air.