Fei Xue

Dark matter is five times more massive than ordinary matter, yet it emits no light. Its nature remains unknown. One candidate is the ultra-light axion particle. Axions could pierce right through the planet, barely noticing atoms.

A magnetic sphere hangs in a vacuum above a cold metal plate. No strings, no stands — only the invisible field of a superconductor keeps it from falling. Give the sphere a gentle nudge, and it starts to oscillate up and down; the frequency of those oscillations depends on how strongly the plate pulls on it.