Not sure I would call that, keeping it simple. Almost nothing in science is simple. Or rather almost nothing is based in direct observation. We cannot see gravity, electromagnetism, electrons, protons or atoms. We have never seen evolution creating species before our eyes. We have never seen continental plates move with out own eyes. Almost everything we know about any science is based on a multitude of indirect observations which we combine to create a model of reality.
Your senses are not perfect nor our human intuition. Everything is in a way filtered. You never see the true reality. You never know anything 100%. But science does not have to. Science creates models which can be expressed mathematically and which make highly accurate observations.
The idea that space itself folds is not just pure guesswork but based on observations which we can make sense of if wake this assumption. I am on thin ice now, but as far as I understand, we see a repetition when we look in any direction in the universe. The light from distant stars, from when the universe was smaller hits us in multiple directions. You can explain that if you assume a sort of bending of space. There is no easy way to explain that if there was a boundary.
I am pretty sure this explanation of mine is not totally correct, but my aim is to covey the idea of how science works and how we know or determine anything.
It might be easier to understand this by reading about the experiments which led people to conclude that there was a big bang, rather than reading about the end theory.