Kordylewski cloud
The Kordylewski clouds, sometimes called the lunar libration clouds, are concentrations of dust that exist at the L4 and L5 Lagrangian points of the Earth–Moon system. They were first reported by Polish astronomer Kazimierz Kordylewski in 1961, who observed the clouds from the Tatra Mountains in former Czechoslovakia. The clouds are likely composed of trapped dust particles from the interplanetary dust cloud. Dust particles remain for decades, forming large, rapidly-evolving bands within the clouds. Eventually, perturbations from the Sun lead to their escape.
Following Kordylewski's discovery, inconsistent observations by other astronomers led to their existence becoming controversial. Attempts to observe the sparse clouds were complicated by their exceedingly dim nature, making them difficult to discriminate against gegenschein and atmospheric airglow even in very dark skies. Observations from the ground, air, and space reported both positive and negative detections, and a 1991–92 encounter from the Hiten spacecraft failed to find the clouds. In 2018, they were tentatively confirmed to exist by a team of Hungarian astronomers through polarimetry. Due to their elusiveness they are sometimes nicknamed ghost moons.