Cooler is a file format that implements genomically-labeled sparse 2D arrays (matrices) with identical axes in HDF5. It is also the name of the Python package that supports the format. Cooler was designed for the storage and manipulation of extremely large Hi-C datasets at any resolution, but is not limited to Hi-C data in any way. Standard file extensions are .cool for single-resolution cooler files and .mcool for files storing multiple resolutions.