Actually, gravity is the weakest of the forces, but the only one that multiplies. Gravity is not a force (although the search for gravitons goes on), but is considered a warp in space/time. Black holes are collapsed stars where the outward pressure of the fusion reaction ends when the fuel is gone and the star collapses in on itself. If it is massive enough (past what is called the Chandresekar limit, about 3.2 solar masses from memory), the force of gravity overpowers the nuclear force that gives atoms their shape/space, and the star collapses into a 'singularity'. The gravity is so strong that light cannot reach escape velocity (hence black hole). The more interesting aspect for most is that when the star collapses it causes a matter shell to super heat and explode; up to 90% of the stellar mass is ejected. The pressures allow exotic materials to form. As a hint, the Earth and Sun are only big enough to produce iron atoms. Anything with an atomic weight over that of iron could not have been made in this solar system. If you own any gold, you own something that (at the atomic level) was created in the death of a star.

Physics is an interesting space... I wish I understood the math.

Oh, the 'other side' of space does exist, but our universe has unique rules of physics (all universes do), so if you 'cross over' it will likely be very unpleasant.