Ultra-diffuse galaxies (UDGs) — galaxies with stellar masses ~107–108 M? spread across half-light radii of 1.5–4.5 kpc, giving central surface brightnesses fainter than 24 mag/arcsec2 — were first systematically catalogued in 2015 (van Dokkum et al. 2015; Koda et al. 2015) and now number in the thousands across nearby clusters and groups. Their formation in ΛCDM remains contested: proposed channels include high-spin halos (Amorisco & Loeb 2016), failed L* progenitors (van Dokkum et al. 2015), tidal stripping in cluster environments (Conselice 2018), and bursty star-formation feedback in dwarfs (Di Cintio et al. 2017). None of these channels predicts any preferred large-scale spatial arrangement of UDGs beyond ordinary clustering with their parent groups.
Yet imaging surveys reveal that UDGs are not isotropically distributed even after group-clustering is accounted for. Within and between groups, UDGs preferentially trace cosmic-web filaments rather than random sightlines — Roman et al. (2019) reported a strong UDG excess along Virgo's tendrils; Lim et al. (2018) and Mancera Piña et al. (2019) found UDG populations strung along inter-group filaments in the Hydra and Coma extensions; deeper surveys (Greco et al. 2022; La Marca et al. 2024) extend the trend to ~10–30 Mpc filament segments. The combined picture — coherent UDG streams along well-defined filament axes, with surface-brightness fading roughly along the same directional gradient — has no natural origin in ΛCDM, where UDG formation channels are local and the cosmic web is irrotational on these scales.