From Science and Technology
ANCIENT Greek philosophers thought the world was composed of four elements: earth, air, fire and water. To explain the heavens, though, many saw a need for something more—quintessence (quinta essentia in Latin), a fifth element. Quintessence was part of the universe, but lay out of humanity’s grasp.
These days, physicists face a similar problem. The material around them, made up of atoms, seems unable to explain what most of the rest of the cosmos is made of. Energy and matter are two sides of the same coin, so a list of universal ingredients must measure both.
The latest research has it that some form of unseen energy, of unknown nature, makes up about two-thirds of that total “energy density”. Of the stuff with mass, which thus counts as matter, inscrutable “dark matter” constitutes another quarter of the universe’s contents. What is left is what humanity perceives as normal matter. But it is far from normal. Every atom of Earth, every star dotting the heavenly vault, every bit of dust between and beyond: together they account for less than 5% of the universe’s recipe.