On shelves at a Humanetics facility in Huron, Ohio, skulls stare from their eyeless sockets, shiny and silver. Around a corner, a rack is filled with squishy, peach-toned arms, legs, torsos and butts.

The Dark Forest theory is one proposed solution to the famed Fermi Paradox, which has bewitched astronomers for more than half a century. To put it concisely, the paradox asks if the cosmos is nearly ...