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We have detected a broad interstellar absorption band centred close to 7700\r{A} and with a FWHM of 176.6$\pm$3.9 \r{A}. This is the first such absorptionband detected in the optical range and is significantly wider than the numerousdiffuse interstellar bands (DIBs). It remained undiscovered until now becauseit is partially hidden behind the A telluric band produced by O$_2$. The bandwas discovered using STIS@HST spectra and later detected in a large sample ofstars of diverse type (OB stars, BA supergiants, red giants) using further STISand ground-based spectroscopy. The EW of the band is measured and compared withour extinction and K I $\lambda\lambda$7667.021,7701.093 measurements for thesame sample. The carrier is ubiquitous in the diffuse and translucent GalacticISM but is depleted in the environment around OB stars. In particular, itappears to be absent or nearly so in sightlines rich in molecular carbon. Thisbehaviour is similar to that of the $\sigma$-type DIBs, which originate in thelow/intermediate-density UV-exposed ISM but are depleted in the high-densityUV-shielded molecular clouds. We also present an update on our previous work onthe relationship between $E(4405-5495)$ and $R_{5495}$ and incorporate ourresults into a general model of the ISM.

Galactic extinction laws: II. Hidden in plain sight, a new interstellar absorption band at 7700 \r{A} broader than any known DIB