Black holes are curved space-time with angular curvature that does not permit light (or any other information) to escape. they are thought to be caused by singularities; two main types of singularities are curvature singularities and conical singularities. Singularities can also be divided according to whether they are covered by an event horizon (eg black hole) or not (naked singularities). According to general relativity, the initial state of the universe, at the beginning of the Big Bang, was a singularity. Both general relativity and quantum mechanics break down in describing the Big Bang, but in general, quantum mechanics does not permit particles to inhabit a space smaller than their wavelengths; therefore one can not intelligently speak of anything "within" a singularity. Another type of singularity predicted by general relativity is inside a black hole: any star collapsing beyond a certain point (the Schwarzschild radius) would form a black hole, inside which a singularity (covered by an event horizon) would be formed, as all the matter would flow into a certain point (or a circular line, if the black hole is rotating). This is again according to general relativity without quantum mechanics, which forbids wavelike particles entering a space smaller than their wavelength. These hypothetical singularities are also known as curvature singularities.
According to what is known so far about black holes,surprisingly, there is nothing inside it. All you would see is a large assortment of objects and particles circling around the black hole which were pulled in due to gravity, this is called an accretion disk. All of these particles continue circling around until they pass through what is called the "event horizon" in which gravity becomes so strong that absolutely nothing will escape it.