Richard W. Pogge

The helium in a party balloon is the very gas that makes it float. But long before balloons, helium appeared in the first minutes after the Big Bang. Its exact abundance in the universe is like a fingerprint: it tells you how many kinds of neutrinos (ghostly particles that pass through walls) exist.

A pebble drops into a night lake, and ripples spread across the water. You can't see the pebble itself, but the ripples give it away. Astronomers use a similar trick when hunting for dark bodies in space: these objects betray themselves by the way they warp the light of distant stars.

In the Galactic bulge, where the light of billions of stars merges into a continuous glow, four cold giants passed unnoticed. You can't see them through a telescope — too far, too dim.