Unknown. Current thought was a singularity, but look into the very new research published regarding black holes, vacuum energy, and dark energy.

https://scitechdaily.com/cosmological-coupling-new-evidence-points-to-black-holes-as-source-of-dark-energy/

https://gagadget.com/en/science/218939-black-holes-contain-dark-energy-that-accelerates-the-expansion-of-the-universe-and-singularities-are-no-longer-ne/