Thank you brothers and sisters for your encouragement.
Many thanks to DCFEVER too.
Kevin Li, zone
The background is the dark side of the wall of the lotus pond.
It is dark in the first place comparing the brightness of the lotus flower under direct sunligh.
There were still some details in the dark background. The way to make it darker in software is :
Layer --> Adjustment Layer --> Level
Slide the bottom left pointer gradually to the right until the dark area is dark enough on the screen.
Happy shooting !