Can quark stars form under an event horizon? This is an excellent question that I have often thought about myself. Personally, I agree with Graeme Heald that quark stars can indeed form under an EH. Why do I agree? Well, quarks are fermions, and fermions must obey the Pauli Exclusion Principle. This results in quark degeneracy pressure at some high curvature of spacetime preventing the formation of the mythical BH singularity. But, I am not an expert. What I can offer is a "Gedankenexperiment" a lá Einstein which might lead to an easily performed "Realexperiment" to settle the issue. Assume it's true. What would likely happen? Suppose a massive star dies and starts its collapse. It crashes through electron DP, then it crashes through neutron DP, but it finally hits the wall of our assumed quark DP after an event horizon is formed. What would happen next? The matter and energy accelerating inward would be forced to bounce off the spherical quark degeneracy pressure. This is just like when a supernova starts its collapse and bounces off the spherical neutron degeneracy pressure, and then explodes outward nearly obliterating itself with stupendous energy. But, in the case of a quark star bouncing off quark DP, it is gravitationally contained within an EH. The quark star cannot obliterate itself outwardly like a supernova. So then what? The bounce off the quark DP boundary accelerates matter and energy outwards to counteract some of the accelerating matter still moving inwards. Thus, the EH jiggles, oscillates, or breathes. It's like a heartbeat. Once the bounce is finished most of the quark star is still within the EH, still gravitationally contained. But the infow-outflow bounce cycle keeps repeating. The only thing that escapes is some high-powered, low-frequency EM radiation. This is a result of the EH breathing in and out. Pretty dynamic stuff, mostly shielded from external view by the EH. But remember, stars and the universe itself are very, very dynamic! ... a singularity is not! So, this is the Gedanken. How do we prove it by a real experiment. Well, look for a cyclical EM signature, mimicking a heartbeat, emanating from the vicinity of a black hole. Or, more generally, look for a cyclical EM signature emanating from a white dwarf (electron DP), a neutron star (neutron DP), or a black hole (quark DP). There is likely some cyclical bounce involved with all these degeneracy pressures. In the case of neutron stars, we've already detected the phenomenon and call such pulsating stars pulsars. So, this long-winded answer may not be the expert answer you were searching for, but at this point in time there are no experts that really do know the answer. Besides, sometimes in physics it's good to think out-of-the-box like that patent clerk did with his ridiculous GR theory. Until presto, it was supported by the data from a solar eclipse experiment in 1919.