Can a plasma be compressed to greater densities than other forms of matter?
If you compress any form of matter enough, chances are good it becomes a plasma in the process, so... "yes"?
The most dense things are black holes, but they don't fall in the typical categories of matter states.
Well if you don't quite go the extreme of a black hole we could look an neutron star interiors, where it is theorised there exist neutron superfluids and possibly even some kind of quark-gluon soup right in the core. Not sure that these are plasmas, exactly...
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