Speaking of 'necessity', here is an interactive survey to see if you have some beliefs or intuitions that bear on the question of necessary concreta: www.necessarybeing.net. The survey showcases arguments relevant to the question, some of which are relatively unknown--and so may be of special interest to metaphysicians who care about this question. Your answers will be recorded for further analysis. Comments and criticisms are welcome.
The topic bears on this question: are necessary existence and concreteness compatible? If we say "no", then we can give the following simple criterion for being concrete: 'x is concrete iff x is not necessary' (unless non-necessary abstracta exist). On the other hand, if concreteness is compatible with necessary existence, then the possibility is open for us to give an ultimate explanation of the existence of non-necessary things in terms of the contingent activities of more basic, necessary things (be they fundamental particles or something else). So, answers to the question seem to have deep implications for fundamental ontology and cosmology.