To discover the identity of dark matter—the elusive stuff that accounts for most of the mass in the universe—researchers have done most of their searching belowground. Part of the answer, however, may lie overhead. Simulations published in this week's Science suggest that if dark matter is relatively "warm," or energetic, star formation in the early universe would have entailed vast filaments full of massive stars, instead of smaller clumps had the dark matter been cold. Researchers say future tests might be able to distinguish the two possibilities.