At least a fraction of that flux is extragalactic origin," Albrecht Karle, a UW-Madison professor of physics and one of the senior authors of the new study, told Live Science. "This was a new discovery."
Those observations also told them something else: The energies of the muon neutrinos, and their numbers, didn't fit well with several models of their origins. The scientists don't address it deeply in their study ("We leave that to theorists," Karle said), but the data appear to show these muon neutrinos are probably not coming from gamma-ray bursts (GRBs), which are highly energetic events in space.
"There are some stringent upper limits of neutrinos from GRBs — we know they don't produce that many," he said.
Similarly, active galactic nuclei don't seem to be the culprit, either, though Karle said it's too soon to say for sure.
Other possibilities are galaxies going through bouts of rapid star formation, or masses of gas and dust that surround black holes at the galactic centers. As atoms get pulled into the maw of a black hole, they slam into each other more often at higher energies. Eventually some produce pions, neutrinos and photons. If that were the case, Karle said, then one would expect a nearly one-to-one ratio of high-energy neutrinos to accompanying photons. But that hasn't been confirmed or refuted yet.
The study is detailed in today's (Aug. 20) issue of the journal Physical Review Letters.