Nouns ending in -er are usually masculine.
Is danger masculine or feminine?
ledanger
The rule behind it
94.6% of -er nouns are masculine
In context
Il est trop stupide pour craindre le danger.
He is too dumb to fear danger.
Lorsqu'ils sont en danger, ils s'enfuient.
When they are in danger, they run away.
Je pense que sa vie est en danger.
I think his life is in danger.