致密 describes something that is tightly structured, with no gaps or flaws — used for physical objects (dense materials, fine-grained stone, tight weave) and also for abstract things like arguments, plans, or behavior that are rigorous, meticulous, or beyond criticism. The sense shifts from 'physically compact' to 'logically watertight' depending on the noun it modifies (材料 'material', 论证 'argument', 结构 'structure').