Cold starlight gives away the trick before theory does. Galaxy disks spin so fast that visible matter should fly apart, yet rotation curves stay flat far from the center, forcing astronomers to postulate a massive invisible halo that outweighs stars by several times and clumps under gravity like any other mass.
The strange part is not the mystery itself but its mathematical neatness. General relativity, through the Einstein field equations, accepts dark matter and dark energy as simple extra terms in the stress energy budget, while observations of the cosmic microwave background and baryon acoustic oscillations fit a smooth model in which ordinary atoms account for only a small minority of the total cosmic density.
What this suggests is a universe that is friendly to equations yet indifferent to intuition. Dark matter behaves like a collisionless fluid shaping gravitational lensing and structure formation, dark energy acts as a near uniform negative pressure driving accelerated expansion, and both slot cleanly into the same compact formulas that describe falling apples and orbiting planets, leaving human perception as the real outlier.
