What does a super massive stars end up as?
A sufficiently massive star, about 20 solar masses or more during its main sequence lifetime, will end up as a black hole (https://en.wikipedia.org/wiki/Black_hole).
For most stars, eventually including our own Sun, the final gravitational collapse of the dead star's core produces a superdense object called a white dwarf -- about a million times as dense as water, as massive as the subscript Syn but no bigger than the Earth.
At this level of density the electrons pile up, forced into higher and higher energy states because of the density combined with the Pauli Exclusion Principle which prevents electrons from accumulating in the limited number of low energy states. The added energy works against gravity to bring the white dwarf into balance, a phenomenon called electron degeneracy pressure.
But it's not foolproof. As Subrahmanyan Chandrasekhar (http://www.britannica.com/biography/Subrahmanyan-Chandrasekhar) discovered, if the stellar core is about 1.4 times as massive as the Sun or more, gravity overwhelms electron degeneracy pressure. The collapse keeps going, until the electrons and protons in matter are forced to merge into a giant lump of neutrons.
The neutrons then produce their own degeneracy pressure to make a neutron star, an object whose density could be hundreds of trillions (USA numbers) times as dense as water -- imagine two solar masses squeezed into the volume we might see in a large mountain on Earth.
But neutron degeneracy pressure fails too when the core is about three solar masses or more, which we can get from a star that initially has 20 solar masses. Now the collapse goes all the way until nothing can escape the all-powerful force of gravity -- a black hole.
And we know they are out there. Besides the indirect evidence in such objects as Cygnus X-1 (https://en.wikipedia.org/wiki/Cygnus_X-1), recently we have found direct proof from the detection of gravitational waves (https://www.ligo.caltech.edu/news/ligo20160211).