The powerful James Webb Space Telescope—the successor to the Hubble Space Telescope—promises insight into profound questions that have dogged philosophers and astronomers for millennia. Brian Greene brings together four scientists who will use the Webb to investigate these very questions.

For nearly a century, evidence has mounted that the gravitational pull necessary to keep clusters of galaxies intact, as well as stars within galaxies from flying apart, requires far more matter than we can see—matter, according to the experts, that has eluded our telescopes, because it does not give off light.

Forget what you think you know about dark matter. After a 30-year search for a single, as yet unidentified, species of dark matter particle that would make up some 25% of the mass of the universe, physicists are starting to consider novel explanations.