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It is highly disputed about the microstructure and evolution of shear bands (SBs) in metallic glasses (MGs) which affect their yielding and plasticity. We reconstruct the nanostructure of SBs in a bent heterogeneous Pd-based MG fiber with synchrotron X-ray nano-computed tomography and perform finite element analysis on their formation and evolution. It is found that apart from plain SBs (PSBs) under small shear offsets, a large amount of hidden SBs (HSBs) of density fluctuations appear inside the MG, which do not induce shear offsets. The average densities of the HSBs are ∼5%–∼25% smaller than the matrix around, and their thicknesses range from ∼100 nm to ∼530 nm. The diversified structures of the HSBs closely relate to the MG's multiscale structural heterogeneity and local strain rate. The HSBs can develop into PSBs that are densified near surface caused by structural relaxation under high temperatures …