Nearly three years ago, a particle from space slammed into the Mediterranean Sea and lit up the partially complete Cubic Kilometer Neutrino Telescope (KM3NET) detector off the coast of Sicily. The particle was a neutrino, a fundamental component of matter commonly known for its ability to slip through other matter unnoticed.

The IceCube observatory in Antarctica, a comparable detector that has been running for more than a decade, has found hundreds of cosmic neutrinos -- but none quite like this one. Some 35 times more energetic than any neutrino seen before, the particle might have shot out from a highly active galaxy -- a blazar -- or a background source (opens a new tab) of cosmogenic high-energy particles that scientists suspect pervade the cosmos.

But those aren't the only possibilities. The day after the KM3NET collaboration announced the detection (opens a new tab), the physicist David Kaiser (opens a new tab) walked into a room full of his colleagues at the Massachusetts Institute of Technology with a bold proposition: What if the monster neutrino came from an exploding primordial black hole?

Such black holes "could form before there were even atoms, let alone stars," said Kaiser, who has been heavily involved in the hunt for these hypothetical objects.

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