According to our current understanding of black holes, it is not possible for the entire observable universe to be inside the event horizon of a single black hole without also being at the singularity.
The event horizon of a black hole is the boundary beyond which no information or matter can escape. Once something crosses the event horizon, it is believed to be inevitably drawn towards the singularity at the center of the black hole, where the mass is infinitely compressed and spacetime curvature becomes infinite.
If the entire observable universe were to be within the event horizon of a black hole, it would mean that the black hole is significantly larger than the observable universe itself. This would contradict our understanding of black holes and the current observations of the universe's large-scale structure.
Additionally, the expansion of the universe poses a challenge to the concept of an entire universe within a black hole. The universe is expanding at an accelerating rate, and this expansion occurs on a scale much larger than individual black holes. This expansion would prevent the universe from collapsing entirely into a black hole.
In summary, based on our current understanding, it is highly unlikely for the entire observable universe to be inside the event horizon of a black hole without also being at the singularity.