Although the image was taken from a dark site, distant light pollution still reflected off dust particles high in the atmosphere. Light pollution made the background brown, because the light was brightest at the red end of the spectrum, then green, then blue. You can tell that from the order of the peaks in the histogram.
You might just be able to see part of the Rosette nebula in the background. It is only just slightly brighter than the background. This is a typical first result after stacking. Processing is needed to boost the nebula against the background.