Astronomy
How the Lyman-Alpha Forest Maps Intergalactic Hydrogen
Quick fact
A single quasar spectrum can contain thousands of Lyman-alpha absorption lines, each from a different hydrogen cloud along the line of sight, spanning billions of light-years.
Why this is interesting
Imagine trying to map an invisible ocean by shining a bright lighthouse beam through it — the distant light gets carved into a pattern of missing colors. Astronomers do exactly this with quasars to map the gas between galaxies.