Active black holes are easy to detect. Their superheated accretion disks emit bright X-rays, and the jets streaming from their poles are easily seen at visible and radio wavelengths. But most stellar-mass black holes are inactive. They either drift through the galaxy alone or orbit a companion star without consuming its material. We can't observe inactive black holes directly. We can only observe their effects on nearby objects. We currently know of three such black holes thanks to the Gaia spacecraft.