<P> 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 observational deductions, but they have not been directly observed and their nature is not well understood . </P> <P> Most standard (baryonic) matter is found in intergalactic gas, stars, and interstellar clouds, in the form of atoms or ions (plasma), although it can be found in degenerate forms in extreme astrophysical settings, such as the high densities inside white dwarfs and neutron stars . </P> <P> Hydrogen is the most abundant element in the Universe; helium is second . However, after this, the rank of abundance does not continue to correspond to the atomic number; oxygen has abundance rank 3, but atomic number 8 . All others are substantially less common . </P> <P> The abundance of the lightest elements is well predicted by the standard cosmological model, since they were mostly produced shortly (i.e., within a few hundred seconds) after the Big Bang, in a process known as Big Bang nucleosynthesis . Heavier elements were mostly produced much later, inside of stars . </P>

What are the 2 most common elements in the universe