When an invisible entity making up 85% of the universe’s mass stumps the greatest scientific minds of our time, awe is an understandable response.

Physicists call it dark matter, a substance they describe as the cosmic glue, the scaffolding, a web that uses gravity to corral, shape and hold together stars, planets and galaxies. Yet nobody knows exactly what it is.

Dark matter’s existence is only inferred from its gravitational effects on visible matter. Together with dark energy—a mysterious force causing the universe to expand at an accelerated rate—they are the biggest scientific mysteries of our time.