Can black holes be a subset of quark stars? If not, what are black holes made of? Can stellar black holes be a category, or subset, of quark stars?
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Strange stars are so called not because they're weird (though they
are) but because the extraordinary pressures within them should turn
some of their constituent quarks into a heavy type of quark named
&quot;strange.&quot; Neutrons and protons are made of two lighter types called
&quot;up&quot; and &quot;down.&quot; (The names were chosen whimsically and have no
particular meaning.)
&quot;It's possible that all the stars we have been calling neutron stars
are in fact strange stars&quot; or hybrids of the two, said Turner. If
strange matter does convert everything it touches into more strange
matter, these stars would consist of the stuff right up to their
perfectly hard-edged surfaces, with nothing but vacuum above. If it
doesn't, the stars could be hybrids with strange-matter cores and
neutron-star outer layers.
from https://skyandtelescope.org/astronomy-news/quark-matter-stars-said-found/

The visible quark stars such as the one depicted would be quark-neutron hybrids. This would conveniently answer the question &quot;what is black hole substance made of&quot;.
If this is certain to not be the case, what (if any) known substance can black holes de made of?
Note, I said a subset - &quot;...strange stars exhibit a low-luminosity but high-temperature bremsstrahlung spectrum, which, in combination with the emission properties of the accretion disc, may be the key signature to differentiate massive strange stars from the black hole.&quot; in https://academic.oup.com/mnras/article/400/3/1632/962273
https://physicsworld.com/a/calculations-point-to-massive-quark-stars/
Black holes are not made of anything... at least nothing in our current understanding of reality.
We understand black holes in terms of General Relativity, the theory of gravity.  In this theory there are some &quot;vacuum solutions&quot;  These are solutions that have no matter at any point in spacetime.  One solution is &quot;flat space&quot;, but another solution is a solution with a singularity in time and space.  This solution was discovered by Schwatzchild, and for long was thought to be a mathematical curiosity but not physically real.
But it was later discovered that when a star collapses under gravity it reaches this state.  All the matter falls towards the singularity and we can't describe what happens later than the singularity in time. From the outside it seems as if all the mass of the star is concentrated at one point, but there is actually nothing inside, just the distorted spacetime.
In General relativity nothing can exist in a black hole. For anything falling beyond the event horizon, all directions lead to the singularity.  It is as unavoidable as &quot;next Wednesday&quot;. No ball of matter could exist. In normal space, two particles can remain separate. Inside a black hole this isn't possible, since both particles end up at the singularity in a finite (and short) amount of time.
Now we aren't certain if this truly describes reality, because we lack a theory of gravity that takes account of quantum mechanics. It might be that the singularity is real. It might be that something happens (with strings or something) that prevents the ultimate collapse to a singularity. Whatever it would be outside our current models of reality.  Not &quot;quarks&quot;.