What do you do when you witness domestic violence. ?

I'm 16 and in the car with my mom. She told me to look to the car behind us, the male was hitting his wife(the driver) my mom said she was crying hystericallly. This went on for about 5 minutes, they were behind us. I asked what to do about it and she said nothing. But I truelly don't believe that,, if I am a witness to it I can't just sit back and watch. I got her license plate, they were argueing a lot and it was a pain to see her crying. Idk I just want it out of my head.