Full Idea
The 'Universe' in mereology is the sum of all objects whatever, a unique individual of which all individuals are part. This is denoted by 'U'. Strictly, there can be no 'empty Universe', since the Universe is not a container, but the whole filling.
Gist of Idea
Universe: the mereological sum of all objects whatever, written 'U'
Source
Peter Simons (Parts [1987], 1.1.09)
Book Reference
Simons,Peter: 'Parts: a Study in Ontology' [OUP 1987], p.15
A Reaction
This, of course, contrasts with set theory, which cannot have a set of all sets. At the lower end, set theory does have a null set, while mereology has no null individual. See David Lewis on combining the two theories.