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

The data availability problem is the challenge of ensuring that the data for all transactions processed on a Layer 2 rollup is publicly available on the Layer 1 blockchain. This is crucial because users must be able to access this data to reconstruct the L2 state and generate fraud proofs (for Optimistic Rollups) or verify validity proofs (for ZK-Rollups).

If the data is withheld, the L2 state cannot be audited or challenged.