Can't Survive
The darkening of a hamster's belly is usually a serious sign, indicating that its health condition has deteriorated to an irreversible extent. Here is a detailed analysis of this phenomenon:

Reasons for the Darkening of the Belly
The darkening of a hamster's belly may be caused by internal decay or the accumulation of gas. This phenomenon usually occurs some time after the hamster's death, such as more than half a month later.
Symptoms Before Dying
In addition to the darkening of the belly, a hamster may also exhibit the following symptoms before it passes away:
Lack of enthusiasm for eating and a decrease in food intake.
Rapid breathing or accompanied by abnormal sounds.
Loss of vitality and becoming more quiet.
Weight loss and the fur losing its luster.
Abnormal behavior, such as continuously spinning in circles or frantically gnawing at the cage.
Treatment Suggestions
If you find that a hamster's belly is dark and accompanied by the above symptoms, it is recommended to take the following measures:
Confirm whether the hamster has died. You can judge by touching its body to see if there is still any body temperature.
If it is confirmed that the hamster is dead, its remains should be disposed of properly to avoid hygiene problems caused by decay.
If the hamster is not yet dead but the symptoms are severe, it is advisable to consult a professional veterinarian, although the possibility of survival is extremely low.
Summary: The darkening of a hamster's belly usually means that it has already died or is about to die, and the possibility of survival is almost zero. At this time, the most important thing is to properly handle its remains and pay attention to the health status of other hamsters to prevent similar situations from happening again.