One of the most important goals for modern cosmology is to reveal the nature of dark matter. Current and future generations of gamma-ray telescopes aim to achieve it by looking for gamma rays from dark matter annihilation. After giving brief overview of the latest observational results with Fermi-LAT, I will focus on galaxy clusters and diffuse gamma-ray background as two potentially interesting target for dark matter searches. For the clusters, I show results of the analysis of gamma-ray data from ~3-yr Fermi telescope, in terms of upper limits on annihilation cross section. Especially, I discuss the importance of effects of baryonic infall that modify the dark matter density profile largely and hence boost the annihilation signals. For the diffuse gamma-ray background, I discuss the average intensity and small-scale anisotropy, and constraints on annihilation cross section from these arguments.
