From this article, it states that
The holographic principle states that the entropy of ordinary mass (not just black holes) is also proportional to surface area and not volume; that volume itself is illusory and the universe is really a hologram which is isomorphic to the information "inscribed" on the surface of its boundary.
So, using the holographic principle, the observable universe has a 2-sphere boundary. This boundary acts like an event horizon of a Schw black hole.
The question: event horizons are coordinate singularities in GR that enshroud physical singularities. Since the edge of the observable universe is the event horizon, then where is the physical singularity "within" the holographic universe?