New Nobel laureates Saul Perlmutter and Adam Riess of the U.S. and Brian Schmidt of Australia contributed to the discovery that the universe is not only expanding but also speeding up.

The finding led to the now widely accepted theory of dark energy, a mysterious force that repels gravity. Measurements show that dark energy accounts for about 74 percent of the substance of the universe.

But more than a decade after the Nobel-worthy find, scientists are still trying to pin down exactly what dark energy is and and thus solve what some experts call "the most profound problem" in modern physics.

(Also see "New Galaxy Maps to Help Find Dark Energy Proof?")

Until dark energy, physicists were convinced that