I saw this once.
Saw it during a very dark night camping at Dinosaur National Monument, the same night I saw the Andromeda galaxy with my naked eye for the first time. The tequila helped.
Gegenschein means counter-glow in German, and that's just what it is. It's a very faint glow, just opposite the sun (yeah, I know, it's night, so the sun is below the horizon; it's just opposite where the sun would be if you could see it). Gegenschein is caused by sunlight reflecting off dust in the plane of the solar system (which is why it shows up directly opposite from the sun). The dust is made of splinters from asteroid collisions, from the tails of comets, from the leftover bits of the formation of the planets. It's quite literally older than the world itself. Gave me goosebumps at the time (as did Andromeda).
Again, the tequila helped.