What Is the “Data Availability Problem” in the Context of Layer 2 Solutions?

The data availability problem is the challenge of ensuring that all the data required to reconstruct the Layer 2 state (i.e. the raw transaction data) is published and accessible on the Layer 1 blockchain. This is critical because without the data, a node cannot verify the state or generate a fraud proof (in Optimistic Rollups), potentially allowing a malicious operator to steal funds.