Spatial modulation (SM) uses a single antenna for transmission each time to avoid synchronization and interferences among transmitter antennas, and differential SM (DSM) is a kind of SM technique that does not need pilot symbols and channel estimation. The original noncoherent detection for DSM is very complicated, so various low-complexity detectors have been proposed. However, they are still too complicated to be applied to DSM with many transmitter antennas. In this paper, low-complexity DSM schemes for dozens or hundreds of transmitter antennas are designed. Two bit-mapping schemes are proposed: one is group arrangement for maximizing data rates, and the other is full mapping so that DSM can be easily detected at the receiver. A low-complexity detection algorithm that is less complicated than all existing fixed-complexity detectors is proposed as well. In addition, two detection strategies for the proposed bit mapping methods are proposed. Simulation results show that the proposed low-complexity DSM schemes have satisfactory error performance.