The bigger the black hole the less dense it becomes...?..So how?

The mass of black hole in solar masses is M = 4.3[9]√(1/ϱ) M⊙, where ϱ is the density of the black hole. The density of the interstellar medium is ≈ 3.5[7] atoms/m³, or 2.1[-33] kg/m³. Ergo,

M = 4.3[9]√(1/2.1[-33]) M⊙

M = 4.3[9]√4.8[32] M⊙

M = 4.3[9] • 2.2[16] M⊙

M = 9.5[25] M⊙

The mass of the Universe is on the order of 5[22] M⊙. About 1/2000 that of a black hole universe. The interstellar medium, however, accounts for ≈ 3.6% of the mass of the Universe. So it only makes up 1.8[-3]% of the mass needed for the Universe to be a black whole.

Would someone please explain to me why anyone would call a mathematical point a black hole? "Black hole" is a description of appearance. If something has zero size it can't be seen. It is the effect of the singularity that would appear as a black hole; the volume within the event horizon.

Edit: The Newtonian, static black hole, an imaginary entity, is the equivalent of "ignoring air resistance" in the piano falls from the top of the Empire State Building questions. If the questioner is an admirer of entrenching pseudo-pedantry he'll be sure skip me for best answer.

Edit: With apologies, my density value of 2.1[-33] seems wrong but I don't have time to work it out right now.

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A black hole considered as a singularity is always infinitely dense. Considered in terms of the volume inside its event horizon, its density is rather imponderable because density is mass per volume and the volume of a sphere whose centre is the location of the singularity and whose surface is the event horizon is not the same as the size of the black hole due to the curvature of space. Therefore, in a sense a black hole has no meaningful density but something for which there is no word as far as i know. The difference between density and this property is similar to the difference between pressure and density.

Interestingly enough, the radius of a black hole with the mass of the Universe works out to be about the current size of the Universe.

Of course with the recent discovery that the Universe is in fact accelerating in its expansion its looking less and less likely that we live in a black hole. In fact, if anything, the Universe bears a similarity to a white hole instead.

Your statement about the sizes of black holes is unfortunately plain wrong. A black hole is a mathematical point - a singularity. It has, therefore, zero volume, regardless of its mass. That is what makes it so extraordinary, and gives it its very peculiar properties. Don't confuse the black hole itself with its event horizon, which is not at all the same thing.

Your first sentence seems to be a statement that your question is based upon.

Why don't you supply the name of the Physicist / Cosmologist who said that?

Because that simply is false.

Your opening premise is flawed. A black hole has no volume and therefore infinite density.

as big as the Oort cloud