New Horizons, drifting beyond Pluto where zodiacal dust no longer fogs the view, measured the optical darkness of space itself, and found it too bright. The LORRI camera's measurements of the cosmic optical background (Lauer et al. 2021, 2022) found a sky brightness roughly twice the integrated light of all catalogued galaxies, a diffuse excess of about 8 nW/m^2/sr unaccounted for by known sources. The measurement is uniquely clean: at 50 AU the dominant foreground contaminations plaguing every Earth-vicinity attempt are absent, making LORRI's photometry the most direct probe of the universe's total optical emission ever performed. Subsequent reanalyses (Postman et al. 2024, with improved Galactic light corrections) reduced the anomalous component substantially, attributing much of the excess to scattered Milky Way light, while leaving a smaller residual whose reality remains contested.
If any excess survives, ΛCDM must source it: candidate explanations span undercounted faint galaxies (requiring steep faint-end luminosity functions beyond HST counts), intrahalo light from stripped stars, direct-collapse black hole populations, and decaying particle contributions, each either ad hoc or constrained by other channels. The episode also feeds the neighboring TeV-transparency accounting, since a brighter optical background deepens expected gamma-ray absorption, tightening the squeeze on the standard photon ledger from both sides.
The standing is a contested residual on a foundational measurement: the cleanest sky-darkness data in history shows the catalogs incomplete at some level between mild and twofold, and forthcoming LORRI campaigns plus JWST deep counts are converging on how much light the universe makes that the inventories miss.