Even on clear moonless nights, the sky is never totally dark.
Shown above is a 360-degree stereographic projection showing the entire night sky near my location about 60 mi (100 km) from Buenos Aires, Argentina.
The pervasive greenish light is a strong airglow display. Airglow is a weak light emission stemming from the chemical reactions involving oxygen, nitrogen, sodium and ozone (chemiluminescence) at altitudes between about 50 to 60 mi (80 to 96 km) above the Earth’s surface. Note that the green color bands, though quite obvious in the camera, weren’t visible to the naked eye.
I was so confused for a second about what that thing in the middle was.