Crack on underside of handlebar I found a crack underneath the handlebar. Is it okay to keep using it? I can't really afford a new one. I'm riding with the cracked bar right now and there's no weird sound and movement on the handlebar.

To make things easier to explain, here is the crack



Not sure what happened. I can feel the crack with my hands and I felt a string like material on the crack. I lent my bike to my brother and he came back with a broken bike. I replaced the broke parts and I didn't check the handlebar until when my friend noticed a strange white line under the bar. I already know it needs replacement immediately.

Please tell me what cause the crack so I might avoid the same problem again in the future.
DON’T

If you are sure (or have strong suspicions) it’s a crack and not just a scratch, stop riding it. 

From your description I’m not sure what’s actually cracked (stem, fork’s steerer tube, frame’s head tube?) but replacing it is much cheaper and less painful than new teeth or sitting in a wheelchair the rest of your life.

Post a picture if you want a more accurate assessment but generally the only components on a bicycle where I’d be okay with imminent failure is anything related to gear shifting or attachments such as bottle cages or mud guards.