> many millions of dollars to anyone who can demonstrate a graph database that can handle a sparse trillion-edge graph.

I wonder why no one has claimed it. It's possible to compress large graphs to 1 byte per edge via Graph reordering techniques. So a trillion scale graph becomes 1TB, which can fit into high end machines.

Obviously it won't handle high write rates and mutations well. But with Apache Arrow based compression, it's certainly possible to handle read-only and read-mostly graphs.

Also the single machine constraint feels artificial. For any columnar database written in the last 5 years, implementing object store support is tablestakes.