Do molecules containing only carbon exist? Why doesn't carbon form bonds with itself to form a molecule? Carbon shows the property of catenation, so why doesn't it form a cyclic molecule such as:
 C = C 
 ‖   ‖ 
 C = C
They do form.  Besides fullerenes, there are carbon molecules with varying numbers of atoms around some stars. Such molecular forms are, of course, less stable than the macromolecular structures we more commonly see under ambient conditions.