The mind clings to the familiar. For example, we are used to the fact that all bodies fall down. They got used to it so much that in England, in Newton's homeland, back in the nineteenth century, a book was immensely popular in society, in which it was "proved" that the Earth is flat, otherwise we would have fallen from it. Since it is flat, it should have an edge. However, Magellan's journey showed that if you sail all the time to the west, you will again sail to Europe, only from the east. So, the Earth is a ball, and the fact that people on the other side walk "upside down" will have to come to terms, even though this is contrary to "common sense".
Well, "common sense" has since somehow come to terms with the law of universal gravitation, but now there is a new task - to understand how the Universe can be limited in volume and thus not have "edges" and something "outside". Well, the best analogy is old games, where, going beyond the end of the screen, some packman, or digger, or snake, or Mario ended up on the opposite side. For them, therefore, the edge of the screen did not exist.
The volume-limited three-dimensional universe is something similar. Imagine: you are in a room that seems to have two doors in opposite walls. You open the door and see the same room and yourself from the back, opening the door in the next wall, behind which you can see another room and another you, and so on. And behind you a door creaked - in fact, the same one, because there is only one door. And this happens not because there are an infinite number of you, but because the Universe is fixated on itself - it's just that the light makes several circles in this Universe before reaching your eyes. If in this our universe to make the speed of light, for example, one meter per second, then you will see yourself in another room with a delay of a few seconds. Now let's add more doors, more precisely, one door to the other two walls of the room. And now - a hatch in the floor and ceiling with the same effects.
Now - let's remove the walls, floor and ceiling! And we will see multiple copies of ourselves through equal intervals of space. Although in fact these copies are as real as your reflection in the mirror - the fact that we see a reflected room in the mirror does not mean that there is another room.
Congratulations! So you found yourself in a universe with a limited volume, but without edges and something "outside". This is just one of the options, toroidal. In a spherical universe, you would see a blurry image of yourself throughout the entire field of view - and, assuming that our viewing angle is 180 °, you would see your nape point-blank, and the crown of your head at the lower edge of your view, the soles of your shoes at the upper edge, and sides - ears. But these are little things.
Why doesn't this happen in our Universe? The fact is that it is expanding, and its rather distant parts fly away from us faster than the speed of light. In general, even if the universe is finite, the light emitted by us or reflected from us simply does not have the ability to return to us. This is a great room option.
Now consider the opposite scenario. We will compress our room without walls. Now we are already uncomfortable in it. Now you no longer fit into it, you are pressed with your nose to your own back of the head, which you see in front of you, and you feel with the back of your head how your nose has pressed against it. Here the room becomes the size of an atomic nucleus … And now we come to the state "immediately" after the Big Bang. "Immediately" is enclosed in quotation marks, because time is also only a dimension of space. So there is not only "outside" the universe, but also "before" the Big Bang. Well, that is, in one of the models.
Something like that…
Promotional video: