dark matter
Imagine you are watching a merry-go-round spinning so fast that the children on the outer edge should be flung off — yet they hold on, as if gripped by invisible hands. Galaxies do something just as strange. The stars and gas at a galaxy's outer edge whirl around so fast that the gravity from all the visible stars and gas is nowhere near enough to hold them in their orbits. Either gravity behaves differently on these huge scales, or there is a great deal of extra mass we simply cannot see. The second idea is called dark matter: a form of matter that has gravity, but gives off and absorbs no light, so our telescopes look right through it.
Dark matter is not one observation but a web of independent ones that all point the same way. Galaxies spin too fast for their visible mass; clusters of galaxies hold together with far more gravity than their stars and gas can supply; the bending of light by gravity (gravitational lensing) reveals mass where no light shines; and the faint ripples in the cosmic microwave background, plus the way galaxies cluster across the universe, only make sense if most matter is dark. Adding it all up, ordinary atoms make up only about 5% of the universe's contents, dark matter about 27%, and the rest is dark energy. So dark matter outweighs everything we can see by roughly five to one.
It is worth being honest about what the name means. 'Dark matter' is a label for a gap in our knowledge: we are confident there is unseen mass tugging on things, but we do not yet know what it is made of. It is almost certainly not ordinary atoms (it is non-baryonic), it moves slowly (it is cold), and it barely interacts except through gravity. Candidates include hypothetical particles like WIMPs, axions, and sterile neutrinos, but despite decades of searching none has been caught. A small minority of physicists argue we should change our theory of gravity instead (MOND). Most cosmologists favor real particles, because dark matter explains many things at once that modified gravity struggles with — but the puzzle is genuinely open.
In the 1970s Vera Rubin measured how fast stars orbit at different distances from the centers of spiral galaxies. She expected the outer stars to move slowly, like distant planets in the Solar System. Instead, their orbital speeds stayed nearly flat all the way out — a 'flat rotation curve' — implying each galaxy is embedded in a vast halo of unseen mass extending far beyond its visible stars.
Flat galaxy rotation curves were among the first strong hints of dark matter.
Dark matter is not 'dark' like a black hole or a dark cloud that blocks light — those are made of ordinary matter. It is invisible because it neither emits, absorbs, nor reflects light at all; only its gravity gives it away.