One time, when I was young, I went on a whale-watching trip with my family. We did see a few whales, though that I remember only vaguely. The thing I most clearly remember about the trip is the guide explaining whale footprints.
A whale footprint is a circle of smooth water left behind after a whale comes up for air and then dives back down. It fascinated me at the time, and has remained a potent image my whole life since. Looking out on an impenetrable blue field, you can find circles of water where nothing is happening -- waters even calmer, smoother, more unremarkable than the rest of the surface of a calm ocean. And from looking at that disc, a small spot of quiet where there should be noise, you can infer the presence of an almost unimaginably large creature, somewhere very close.
In my mind this is a metaphor for almost every kind of knowing. It's what it means to learn by observation. It's how difficult problems are solved. The use of spectrometers to determine the chemical compositions of stars, Thomas Picketty's use of French and British literature to chart economic histories, quantum physics, critical theory, and so on, and so on, and so on. They're all whale footprints. Finding quiet where there should be noise, noise where there should be signal.
Even the most elusive of phenomena leave strange shapes behind them in their path, and those shapes render the unknown, in some way, knowable.