Which sources produce the diffuse TeV to PeV neutrino flux?
In plain words
The IceCube detector at the South Pole sees a steady glow of very energetic neutrinos from all directions, but known source types explain only a small part of it.
Precise statement
The IceCube diffuse astrophysical flux (per-flavor $E^2 \Phi$ of about $1e-8\,\mathrm{GeV}\,\mathrm{cm}^{-2}\,\mathrm{s}^{-1}\,\mathrm{sr}^{-1}$ near 100 TeV, spectral index $\sim 2.4\ \text{to}\ 2.9$) is not accounted for by gamma-ray blazars (stacking limits of order 10 percent) or by the Galactic plane (of order 10 percent near 30 TeV). Identify the population or populations producing most of it, including gamma-ray-dark sources such as Seyfert coronae, consistent with the isotropic gamma-ray background. Answer: source classes with flux fractions summing to the observed flux.
What would settle it
Point-source detections at $5\sigma$ of several members of each candidate class with next-generation telescopes (IceCube-Gen2, KM3NeT, Baikal-GVD) and the summed population flux.
Status in the literature
Unverified note
2022: IceCube reported $4.2\sigma$ neutrino emission from the Seyfert galaxy NGC 1068; hints from other X-ray-bright Seyferts followed in 2024-2025.