Memory storage mechanism is genuinely unknown

The standard story that memory resides in synaptic connectivity and learning is the modification of those connections (work for which Kandel won a Nobel) is challenged by critics like Randy Gallistel, who argue neuroscience cannot even explain how the brain stores a single number like 17, because human memory needs both content-addressability (like word association) and address-addressability (digital location-based retrieval and variable-based computation), which the synaptic story does not adequately deliver.