Abundance of chemical elements

The elements are only a small part of the content of the Universe. By "elements" is meant ordinary (baryonic) matter made of protons, neutrons, and electrons. 
Observations suggest that "only 4.6% of the universe's energy makes up the visible (baryonic) matter of the universe". That includes the energy ("E" = "mc"2 ⇔ "m" = "E" / "c"2) in 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 are "believed" to exist on the basis of scientific theory and inductive reasoning. They are based on observations, but they have not been directly observed. Their nature is not well understood.
Most standard (baryonic) matter is found in intergalactic gas, stars, and interstellar clouds, in the form of atoms or ions (plasma). Degenerate forms are found in extreme settings, such as the high densities inside white dwarfs and neutron stars.
The inner planets of the Solar System are unusual because they have a high amount of heavier elements. These heavier elements were formed in earlier supernovae.