Permitting little if any light to pass through because of denseness of matter:
— Dense smoke.

synonyms: heavy, impenetrable.

Roget 72: assembled etc. v.; closely packed, dense, serried, crowded to suffocation, teeming, swarming, populous; as thick as hops; all of a heap, fasciculated cumulative.   

Roget 321: dense, solid; solidified etc. v.; caseous; pukka^†; coherent, cohesive etc. 46; compact, close, serried, thickset; substantial, massive, lumpish^†; impenetrable, impermeable, nonporous, imporous^†; incompressible; constipated; concrete etc. (hard) 323; knotted, knotty; gnarled; crystalline, crystallizable; thick, grumous^†, stuffy.    undissolved, unmelted^†, unliquefied^†, unthawed^†.    indivisible, indiscerptible^†, infrangible^†, indissolvable^†, indissoluble, insoluble, infusible.    ... show more

Roget 519: unintelligible, unaccountable, undecipherable, undiscoverable, unknowable, unfathomable; incognizable^†, inexplicable, inscrutable; inapprehensible^†, incomprehensible; insolvable^†, insoluble; impenetrable.    illegible, as Greek to one, unexplained, paradoxical; enigmatic, enigmatical, puzzling (secret) 533; indecipherable.    obscure, dark, muddy, clear as mud, seen through a mist, dim, nebulous, shrouded in mystery; opaque, dense; undiscernible etc. (invisible) 447^†; misty etc. (opaque) 426; hidden etc. 528; latent etc. 526.    indefinite, garbled etc. (indistinct) 447; perplexed etc. (confused) 59; undetermined, vague, loose, ambiguous; mysterious; mystic, mystical; acroamatic^†, acroamatical^†; metempirical; transcendental; occult, recondite, abstruse, crabbed.    inconceivable, inconceptible^†; searchless^†; above comprehension, beyond comprehension, past comprehension; beyond one's depth; unconceived.    inexpressible, undefinable, incommunicable.    unpredictable, unforeseeable.    ... show more

Hard to pass through because of dense growth:
— Dense vegetation.

synonym: thick.

Having high relative density or specific gravity.

Slow to learn or understand; lacking intellectual acuity:
— So dense he never understands anything I say to him.

synonyms: dim, dull, dumb, obtuse, slow.

Polish: tępogłowy, tępy