Subtract the Galaxy's own emission and every cataloged radio source from the low-frequency sky, and a glow remains that should not be there. The ARCADE 2 balloon experiment measured the absolute radio background and found an amplitude several times higher than the integrated emission of all known source populations, with a spectral slope that no simple extrapolation of the counted sources reproduces (Fixsen et al. 2011; Seiffert et al. 2011).
The excess is squeezed from every side. Making it with more of the known sources, fainter radio galaxies and star-formers below current detection limits, overproduces the source counts that deep surveys actually measure; making it with diffuse processes runs into gamma-ray constraints, since most mechanisms energetic enough to produce the radio glow co-produce gamma rays the Fermi background cannot accommodate; and dark-sector explanations, annihilating or decaying particles tuned to radiate at radio frequencies, are constructions with no independent motivation (Fornengo et al. 2014; Cooray 2016). The 21-cm anomaly reported by EDGES sharpened the stakes: an enhanced radio background at high redshift is one of the few clean explanations for that absorption depth, requiring the excess to exist early, which the standard source inventory cannot arrange.
The standing is a stubborn budget violation in the sector where budgets are usually clean: radio astronomy counts its sources well, and the counted sources do not add up to the measured sky.