**2.3 Comparison with theory**

Structures *in* the universe are used to deduce the structure and the
history of the universe. Some methods employed are *statistical* applied
to ensembles of particles, assumed to be indistinguishable. Studies of
*time-dependent particle properties* are required in order to either
ascertain the validity of this assumption or to correct the measured
properties appropriately.

World models needed for the comparison with observational results will
be described in Sect. 3. The simplest
models require that the
clustering properties of matter disappear on sufficiently large
scales. The observational study of large-scale structures serves to
determine the *types, scales* and *degrees* of clustering, the
possible *causes* of clustering and the *scale size* above
which uniform distribution becomes apparent (or not).