My dog didn't stop barking because of something that scared him in the yard in dark what was it?

We know that dogs can see things we can't see, When a dog starts barking he may be observing something noteworthy, and wants you to look. It could be another smaller animal in the yard that you didn't see. maybe your dog saw another animal that you didn't notice or someone else was teasing him. Your dog could've been barking out of boredom or for attention.
Some animals act differently when before it rain or when it gets cold including dogs. There are many different reasons why your dog could be barking for no apparent reason. We simply can't see everything they can or like I said before, he might just be bored.