The elements – that is, ordinary (baryonic) matter made of protons, neutrons, and electrons, are only a small part of the content of the Universe. Cosmological observations suggest that only 4.6% of the universe's energy (including the mass contributed by energy, E = mc² ↔ m = E / c²) comprises the visible baryonic matter that constitutes stars, planets, and living beings. The rest is thought to be made up of dark energy (68%) and dark matter (27%). These are forms of matter and energy believed to exist on the basis of scientific theory and inductive reasoning based on observations, but they have not been directly observed and their nature is not well understood.