About how much of the universe's total energy is the ordinary matter that makes up stars, planets and people?

Answer

Planck satellite results put ordinary matter near 5%, dark matter near 27% and dark energy near 68%.

Context

The 5% figure comes from fitting a cosmological model to the pattern of tiny temperature ripples in the cosmic microwave background, mapped most precisely by ESA's Planck satellite. In that fit, ordinary (baryonic) matter makes up about 5% of the universe's total energy content, dark matter about 26% and dark energy about 69%. The numbers are model-dependent and carry small uncertainties, so they are usually quoted as round values.

Dark matter is inferred from gravity alone. Galaxies rotate faster at their edges than their visible stars and gas can explain, galaxy clusters bend light more strongly than their visible mass allows, and the growth of cosmic structure matches a universe with extra unseen mass. No experiment has yet identified a dark-matter particle. Dark energy is the name given to whatever drives the accelerating expansion first seen in distant supernovae in the late 1990s; its nature is unknown.

A common misconception is that the 5% is all stars. Most ordinary matter is not in stars at all: a large share sits as hot, thin gas between galaxies and in galaxy clusters, which made it hard to find, and surveys of this gas helped close the long-standing 'missing baryon' accounting problem. Another misconception is that dark matter and dark energy are the same thing; dark matter clumps and slows expansion, while dark energy is smooth and speeds it up.