The gegenschein is a faint misty patch of light in the night sky, visible to the naked eye in very dark conditions. It can be seen in this all-sky image, just above the middle and a little to the left.
The annotated image can help you to find it, and you can use averted vision (i.e. look a little to the right or left of the object, but focus your attention at the spot where you expect to see it). The image was a by-product of an observation I was making of a comet (comet Schwassmann-Wachmann 1) on February14, 2023, and was taken at the Utah Desert Remote Observatory in the USA.
(Top) Contrast-enhanced version of an all-sky image taken on February 14th, UDRO. (Above) Annotated version showing the location and rough extent of the gegenschein, the four cardinal directions, the plough and Leo (just above the corresponding label, though these constellations are upside down compared with the usual view), and the star Regulus. The Milky Way curves round the western horizon.
The gegenschein is caused by dust in the solar system. Formed largely from asteroidal collisions, this dust fills a galaxy-shaped distribution close to the plane of the solar system. It is lit by the sun, but the scattered sunlight is especially concentrated in the direction back towards the sun, because of a phenomenon called “backscattering”. Therefore the dust is especially bright when viewed from the direction of the sun, and so the gegenschein appears in the sky opposite the position of the sun. It is not thought that there is a particular concentration of dust at the location of the gegenschein. The word “gegenschein” itself is a composite of the German words “gegen” (opposite) and “schein” (glow).
The dust becomes visible again nearer the sun, in the phenomenon called the “zodiacal light”. Furthermore, between the gegenschein and the zodiacal light the dust is dimly visible in a strip of light called the “zodiacal band”, though these two features are not visible in this image. The zodiacal band is fainter still than the gegenschein, which is fainter than the zodiacal light, and this in turn is fainter than the brighter parts of the Milky Way. Because it is so faint, the gegenschein is visible to the naked eye only from very dark skies. I have seen the zodiacal light on two occasions, but have never seen the gegenschein. Alan Pickup, who writes his monthly sky guide for this website, reported that he has seen it from southern Spain and from Mauna Kea (Hawaii). He points out that this is a good time of year to see it, as it falls in a relatively starless part of the sky. (The image shows that the gegenschein is not far from the zenith, while the Milky Way is near the horizon.)
Somewhere inside the gegenschein is the James Webb Space Telescope! (It also lies opposite the sun.)