Hypothetical particles (6)
I believe the answer is:
quarks
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">hypothetical particles</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for quarks that I've seen before include "Hypothetical fundamental particles" , "Particles in physics" .)