The gravitating mass of galaxies and galaxy clusters can be accounted for almost entirely by the stars and gas that astronomers can observe directly.

Astronomy begins with what gives off or blocks light: stars, glowing gas, dust, planets, and galaxies.

It was therefore natural to treat visible matter as most of the material in the universe.

Gravity told a different story.

In galaxy clusters, galaxies move too quickly to be held together by the gravity of the visible material alone. Within individual galaxies, stars far from the center also orbit faster than expected if most of the mass followed the visible light.

The simplest conclusion was that much more gravitating matter was present than telescopes could see. This unseen component became known as dark matter.

Gravitational lensing supplied another way to detect it. Mass bends the path of background light, so astronomers can map the gravitational mass of a galaxy or cluster even when that mass emits no light.

Cosmology then broadened the inventory again. Measurements of distant supernovae showed that the expansion of the universe is accelerating. Together with the cosmic microwave background and large-scale structure, these observations led to the standard model in which ordinary matter is only a small fraction of the total cosmic energy density. Dark matter contributes much more, and dark energy contributes the largest share.

The names do not mean that the underlying physics is fully understood. The nature of dark matter and dark energy remains one of the largest open problems in physics.

The robust correction is simpler: the matter we see shining is not most of what gravitational and cosmological measurements require.