How Supermassive Black Holes Were Discovered
nautil.us
nautil.us
So despite all the data black holes have still never been observed.
What might prevent a black hole from forming? Time dilation is a top candidate (leading to what's called "frozen stars", or "collapsed stars"). From our POV the black hole never quite forms. (And yes I know, infalling matter sees a black hole - but we don't.)
Other options would be quark degeneracy leading to quark stars (similar to neutron stars). Is that strong enough to stop a black hole? It's unknown.
Sure we do; a black hole is defined by the presence of a horizon, and we will certainly see infallers suffering one-way dimming and redshifting into undetectability. Nobody outside will see what has become of any of the original collapsed matter, either, barring failure of the censorship conjectures and ignoring evaporation. (Infalling matter, however, will collide with whatever state matter inside takes, whether that's classical GR's gravitational singularity or some degenerate form of matter that avoids ultimate collapse.)
> "frozen stars" ... "collapsed stars"
Right: collapsed to the point where all the material has become too dim and redshifted to observe; frozen to the point where outside observers cannot track the evolution of the material inside the horizon.
> quark stars (similar to neutron stars)
Similar in that the critical radius is inside the object, rather than the reverse, and that the critical density is not reached within that radius because of terms offsetting internal pressure. Earth is similar to neutron stars in that way too.
An ultrarelativistic fermion gas of quarks inside a horizon is still a black hole to any outside observer; the observables from abolishing the gravitational singularity using such a gas (or anything similar; cf. preon stars etc.) are only revealed, if at all, during evaporation.