Most of the matter in the universe has never actually been directly observed

We infer the existence of dark matter only indirectly — from how galaxies rotate (too fast for the visible matter alone to hold them together), how light bends around clusters of galaxies, and from maps of the radiation left over from the Big Bang.

All of these independent lines of evidence point consistently to the same conclusion: the universe must contain several times more matter than what we can actually see in the form of stars, planets, and gas.

Despite decades of attempts in underground laboratories built specifically to catch dark matter particles, no experiment has ever registered it directly or established conclusively what it's actually made of.

That makes dark matter both one of the best-"confirmed" phenomena in cosmology and one of the biggest open mysteries in physics — we know something is there, we just don't know what.