  We present a novel Diffusion Offline Multi-agent Model (DOM2) for offline
Multi-Agent Reinforcement Learning (MARL). Different from existing algorithms
that rely mainly on conservatism in policy design, DOM2 enhances policy
expressiveness and diversity based on diffusion. Specifically, we incorporate a
diffusion model into the policy network and propose a trajectory-based
data-augmentation scheme in training. These key ingredients make our algorithm
more robust to environment changes and achieve significant improvements in
performance, generalization and data-efficiency. Our extensive experimental
results demonstrate that DOM2 outperforms existing state-of-the-art methods in
multi-agent particle and multi-agent MuJoCo environments, and generalizes
significantly better in shifted environments thanks to its high expressiveness
and diversity. Furthermore, DOM2 shows superior data efficiency and can achieve
state-of-the-art performance with $20+$ times less data compared to existing
algorithms.
