After the Big Bang, the universe was hot and dense. In clumps of dark matter, invisible stuff felt only through gravity, its particles, colliding, annihilated, giving birth to powerful radiation. This hidden heat, like an invisible furnace, warmed gas clouds from within. Colossal dark stars flared up — puffy, sprawling, shining brighter than entire galaxies. When the inner flame died, the star collapsed into a black hole. This mechanism, as Vera Rubin and Fritz Zwicky believed, could have spawned the supermassive holes at the centers of galaxies. Now astrophysicists have modeled the collective light from these giants over the entire history of the universe's expansion. It turns out its faint gamma glow, collected by the Fermi satellite, can be teased out from the background — if the dark matter particles aren't too heavy. This offers a chance to feel out the invisible. And here's an unexpected twist: if such a star were nearby, we'd see it in an ordinary telescope, a bright dot with a bizarre color pattern.