Some 23.3 percent of the cosmos exists in the form of dark matter, while 72.1 percent — according to the latest and most precise measure — is dark energy. The nature of dark matter and dark energy remains a mystery. But surprisingly, so does much of the universe’s normal, or “baryonic,” matter — the stuff, like you and me, made of protons and neutrons.
The curious matter of the missing matter
Scientists still can't account for about 40 percent of the stuff of stars, planets, and people.