Metallic glasses (MGs) are known to be structurally heterogeneous at the nanometer (nm) scale. In addition, elastic property mapping has indicated the presence of at least an order-of-magnitude larger length scales, of which the origin continues to remain unknown. Here we demonstrate the existence of an elastic decorrelation length of the order of 100 nm in a Zr-based bulk MG using spatially resolved elastic property mapping via nanoindentation. Since compositional modulations, sufficiently large to account for this elastic microstructure, were not resolved by analytical scanning-transmission electron microscopy, chemical phase separation such as spinodal decomposition cannot explain their presence. Instead, we argue that the revealed long-range elastic modulations stem from structural variations affecting the local density. These emerge during solidification and are strongly influenced by the cooling constraints imposed on bulk MGs during the casting process.