What happens if you hit someone from the back your car has no damage but the other car has slight damage and the police were not called?

As the shunter, it is assumed, in normal circumstances, that you are At Fault. Either offer to pay for the repair yourself, or exchange insurance details and let your companies sort it out. Sometimes it's cheaper in the long-run to pay for the damage repair and protect your insurance record. If, however, the shuntee wishes to go the insurance route, you have no choice but to agree