You don't understand physics, simple or otherwise. space expands metrically. More space appears between every point in the universe. That's what the expansion is It is space being 'generated'. Not something moving into a pre-existing area. I'm sure that you can't understand it, and so will proclaim it can't be so. Given the struggle to explain that mass and weight are different properties, there's no way you could grasp this.
No, that is what not is happening, you have it wrongly perceived , has the matter expands, it gains more distance from us and more distance apart, it is travelling through the space, like being inside a football and the walls of the football was expanding out, except space has no walls, that is a mistake in visual perception to think that space is expanding, when it is the matter expanding into space, we can clearly see this by the Doppler red shift of matter that is expanding.
''According to the Big Bang model, the
universe expanded from an extremely dense and hot state and continues to expand today'', being or having been enlarged or extended, in particular, to enlarge or extend anything there has to be a volume of space equal or greater than the expansion, this is general Physics and true.
Space is not generated and the logic says even it were , which it is not, that the generated space would need space to be generated in, to say space is generated is to suggest that in some way matter will be displaced to generate space.