"The missing link in cosmology is the nature of dark matter and dark energy," Hawking said Tuesday while delivering a lecture on the origin of the universe at the California Institute of Technology in Pasadena.
He cited data from telescope observations indicating "normal matter is only 5 percent of the energy density of the known universe; 27 percent is dark matter, 68 percent is dark energy," the Los Angeles Times reported.
Hawking -- the author of a popular book on the origin of the universe, "A Brief History of Time" -- made his considerable reputation studying black holes, massive structures at the heart of galaxies that swallow everything around them, even light.
Dark matter and dark energy comprise one of science's greatest riddles. Dark matter can't be seen or felt directly, but scientists infer its existence because its gravity can explain what holds spiral galaxies together.
Dark energy, physicists believe, would explain why the universe is expanding at an ever-growing rate instead of collapsing under its own gravity.
Confirmation of either -- or both -- would represent a breakthrough in theoretical physics, Hawking said.
"There have been searches for dark matter, but so far no results," he said.