I mean, Schwarzschild radius shows that for a medium of constant density (and on a large scale, Universe is fairly uniform) there is an upper limit of a radius of a ball comprised of said medium above which it will form an event horizon.

https://en.wikipedia.org/wiki/Schwarzschild_radius#Calculating_the_maximum_volume_and_radius_possible_given_a_density_before_a_black_hole_forms

Which means that an infinite universe of non-zero density is either a bloody paradox (spend a minute deciding where exactly event horizons should form and whether there will be gaps), or our understanding of gravity and spacetime breaks on ginormous scales just as it does on micro ones.

PS: I have seen no physicists talk about this, so there’s a good chance that there’s a simple resolution to the problem and I’m just stupid.