What if the other insured never contacts the insurance?

I was rear ended on the freeway a couple of weeks ago. The driver was not the owner of the car but I was able to get insurance information from the owner of the vehicle and witness information also. I then contacted my insurance but since I don't have full coverage, they helped me contact the other insurance. Its been about 3 weeks now and the insured has still not contact their insurance yet. What would happen if they never contact. I have uninsured/under insured coverage, will that help? Seems like the driver was a little fishy because he seemed like he wanted to run.