what is darknet and why is it needed for YOLO object detection? what is darknet and why is it needed for YOLO object detection ? I read that its a neural network written in C , but why is it needed for YOLO object detection  when we  have lot of machine learning framework,api like tensorflow,keras,pytorch .

Im trying to train yolo from git code and i could see they are using tensorflow/keras as well but not sure why darkenet is used initially for traning yolo  .

darknet/yolo algorithm
https://pjreddie.com/darknet/ is their website...

I cite : 

"Darknet: Open Source Neural Networks in C

Darknet is an open source neural network framework written in C and CUDA. It is fast, easy to install, and supports CPU and GPU computation."

As to why they used that, well it's open source and in C, which are good points and seems to be performant (see the graphs in your link and associated paper). But the main point seems to be about history. The darknet project seems to have started in 2014. Were tensorflow / keras available and perfromant at this time in the first place ? Even if they were why should they use keras / tensorflow ?