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 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, the physicist David Kaiser 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.

The idea that the neutrino came from a primordial black hole is a long shot; Kaiser said he was “half-joking” when he suggested it. But in the absence of a definitive explanation, it remains intriguing, not least because the existence of primordial black holes could mean they play a role in dark matter.

So the question is, did we just spot one?

In a Split Second

The idea of primordial black holes was first proposed in 1966 by the Soviet physicists Yakov Zel’dovich and Igor Novikov, and it was cemented by the British astrophysicist Stephen Hawking in 1971. Hawking and his student Bernard Carr, of Queen Mary University of London in the U.K., then worked out the concept of primordial black holes in detail in 1974.

A primordial black hole, or PBH, is loosely defined as a black hole that formed in the first split second of the universe. The hypothesis goes that during the rapid expansion of space, there might have been spikes in the density of space-time that were so high that they would have collapsed into black holes. These black holes would have spanned a range of masses, depending on the size of the spikes. Some could have been as small as an atomic nucleus.