Could the known universe be inside a black hole and it's curvature really be the event horizon of the black hole?
No. The metrics look very differently.
- The metrics of the the Universe is determined by the FLRW metrics. Its spacelike sections are planar.
- The spacelike sections of the spacetime around a Schwarzschild (non-rotating) BH is spherical.
More elementarily formulated: the black hole is like a static sphere. The Universe is like an expanding sheet. However, this is what we currently see, matched by our current models. These are not so strong facts, like the density of the iron, what anybody could measure on his desk. I think the correct explanation is roughly this: "it doesn't really looks so, but no one knows".