I read Kip Thorne’s book Black Holes and Time Warps when I was in high school, and it really got me interested in some of the bizarre situations that Physics predicts about the universe. Thorne’s book is autobiographical, going through (mostly in order) the kinds of fields and studies he did through his career. One of the interesting things he would do is describe some outrageous construction allowable by theory but practically impossible, and then step by step go through what it take to be able to engineer it in the real world.
One such example was wormholes. A wormhole is a tunnel through spacetime – the way I always think about it is as follows: imagine space is a rubber sheet/mat, and objects like planets and stars are sitting on top of the sheet, deforming it according to each object’s mass. Very massive objects deform it by a lot. A singularity, the center of a black hole, is a literal puncture through the sheet because a huge amount of mass was pinpoint focused when a massive object collapses under gravity. Now, suppose that the sheet was folded slightly, so that two locations on the sheet were separated by not much actual distance if you hopped across the void. A wormhole is what you get when you punch through the two sides of the sheet and join them in the “middle” (void/hyperspace).
Wormholes are unstable, and it’s easy to see why. The “tunnel” of a wormhole is actually spacetime, so it is responsive to mass. Just like all other spacetime, it bends in the presence of mass. When any sort of mass (including microscopic particles, essentially omnipresent) traverses the tunnel, it pulls the walls in and causes collapse. This is why wormholes need something called “exotic matter” – such hypothetical substance would have a negative gravitational aspect, meaning it would be gravitationally repulsive. Does such material exist? Maybe.