For nearly a century, observations of the planet Venus have noted strong anomalous absorption of solar radiation from 0.2-0.5 micrometers (covering spectra from ultraviolet to blue), primarily in the upper cloud layer above 60km. Less prominent absorption in the lower and middle clouds has been accounted for via sulfur dioxide, but the absorption of 90% of UV-blue in the upper clouds has resisted explanation across decades of research.

"Perhaps the most exciting phenomenon of all is in the atmosphere, just above the super-rotation layer. Here, at about 80 kilometres altitude, something is absorbing ultraviolet wavelengths of light. There is no obvious explanation of these mysterious absorption patches. However, some scientists believe that they could be acid-eating microbes using ultraviolet light in some alien photosynthesis process."

ESA, "No shortage of mysteries on Venus", 28/11/2002

"Comparisons between models of the expected spectra and observations reveal unexplained absorption in the near-UV to blue region of the spectrum. While many candidates for this “unknown absorber” have been proposed over the years, none have been conclusively demonstrated to match the physical and optical behaviour observed (Pérez-Hoyos et al., 2018, JGR Planets, 123).
[…] The unknown absorption was first observed close to 100 years ago, yet the mystery of its cause remains unsolved. More representative spectra of ferric chloride and a greater understanding of its behaviour in the atmosphere of Venus are critical to advancing the identification of the unknown absorber. "

Egan, J., James, A., Plane, J., Murray, B., and Feng, W.: Laboratory experiments to constrain the identity of Venus’s unknown UV absorber, Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-76,6, 2022.

"One of the most intriguing, long-standing questions regarding Venus's atmosphere is the origin and distribution of the unknown UV absorber, responsible for the absorption band detected at the near-UV and blue range of Venus's spectrum.
[…] disulfur oxide and dioxide disulfur provide the best agreement with our results."

Pérez-Hoyos, S., Sánchez-Lavega, A.,García-Muñoz, A., Irwin, P. G. J.,Peralta, J., Holsclaw, G.,…Sanz-Requena, J. F. (2018). Venus upper clouds and the UV absorber from MESSENGER/MASCS observations. Journal of Geophysical Research: Planets Planets, 123, 145–162.

The venusian atmosphere shows strong absorption of solar radiation between 0.2 and 0.5 μm as observed since the early 20th century (Wright, 1927, Ross, 1928). Many candidates have been proposed to explain the nature of UV contrast features. For instance, hydrobromic acid (Sill, 1972), amorphous sulfur (Toon et al., 1982), disulfur monoxide (Na and Esposito, 1997), gaseous Cl2 (Pollack et al., 1980) or even living sources such as bacteria (Schulze-Makuch and Irwin, 2002, Schulze-Makuch et al., 2004, Fritzius, 2008) have all been suggested as potential UV absorber. Sulfur dioxide (SO2) is a strong UV absorber but SO2 could only explain the absorption of region between 0.2 and 0.32 μm, while absorption of longer wavelengths requires another absorber that is not identified yet (Esposito et al., 1997, Bertaux et al., 1996, Moroz et al., 1985).

Molaverdikhani, K., McGouldrick, K., & Esposito, L. W. (2012). The abundance and vertical distribution of the unknown ultraviolet absorber in the venusian atmosphere from analysis of Venus Monitoring Camera images. Icarus, 217(2), 648–660.

As V.A. Krasnopolsky of the Catholic University of America and Moscow Institute of Physics and Technology, the first person to construct a photochemical model for the atmosphere of Venus above the cloud layer, concludes in a 2021 paper in Icarus, “there is no general agreement on the nature of the UV absorber in Venus, and thus this remains as one of the most intriguing open questions in planetary atmospheres.”

Sheehan, William (2021). Unveiling the clouds of Venus. Astronomy, December 2021

CUVE, the CubeSat UV Experiment, a proposed NASA mission concept for a small satellite to investigate this question directly.