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answers:
Michael say: No air just space for objects to enter & fill. Mike
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poornakumar b say: none knows
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Dixon say: mass, charge and spin.
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Neb say: Certainly no one knows for sure.
However, if you believe that Einstein's general relativity still holds at the center of a black hole AND you believe in the strong energy condition (e.g. no negative pressure, no negative mass/energy, and no quantum effects that will generate a 'repulsive' gravity), then the Penrose singularity theorem holds true. The Penrose singularity theorem asserts that all geodesics within the event horizon will terminate at a point. Additionally, the curvature invariant calculated directly from general relativity (kind of a total curvature) becomes infinite at R = 0. Since it is an invariant, it is the same in all coordinate systems and therefore is not a coordinate system singularity (such as at the event horizon in the Schwarszchild coordinates).
If you believe that general relativity needs to be replaced by a true quantized gravity theory, you will also likely believe that a quantum gravity theory will 'fix' the singularity problem of general relativity. We don't have an accepted theory yet.
Lastly, there is a complicated modification of general relativity called Einstein-Cartan theory. Normal matter/energy of general relativity generates a torsion-free curvature of space time (think of a tangent plane sliding along a path on a sphere. The tangent plane will not 'twist' as it moves along the path in normal relativity). Intrinsic quantum spin is another source of space-time curvature, and Einstein-Cartan asserts this creates a torsion component to space-time curvature (the tangent plane twists along a path). Massively dense quantum spins generates the torsion component of space-time which creates a repulsive gravity which overcomes the compression due to strong energy components, preventing a singularity from forming. Unfortunately, this space-time torsion is directly coupled to spin so it doesn't show up in vacuum solutions (such as Schwarszchild), therefore there is no way to tell if it true. In addition, there is no way to duplicate the quantum spin density required in a lab. Arguably, it is a more complete theory than non-torsional General Relativity though and it 'fixes' General Relativity without even needing a quantized gravitational field.
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Nyx say: The remains of whatever fell in past the event horizon.
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