I think people have a very hard time understanding something that is endless/limitless.

We tend to think of things based on our experiences, which are limited, of course. But, there really is no reason to say that a limitless universe cannot be real.

The best "evidence" that the universe is not limitless is that we can observe it has changed with time. For instance, supernovas produce more heavy atomic nuclei, so the composition of the universe that we observe must be changing. But, we have no way of knowing for sure if it can change back in some cyclic fashion, either in its entirety or in parts that are undergoing opposite phase changes at different locations. We just can't see enough of the universe to know what it is all doing, right "now" or in the past. Yes, we have theories, but they are built on a biased perception from our evolution as observers and problem solvers based on extremely limited experiences in an extremely limited environment.

As pointed out in previous posts, mathematics uses "infinity" as another word for "undefined". It is not something that can be manipulated by algebra to solve for a value. But, that is just mathematics - it is not proof that something real, like the universe, cannot be infinite.

As for infinitesimal, that is even worse to ponder. But, we do believe in black holes. What is the matter that goes into a black hole like in there, inside the event horizon that we cannot see into? Is it some super-dense, finite ball of matter with a finite radius? Or, is it a singularity - infinitely small - like some people postulate the beginning of the universe was at the "big bang"? Perhaps the matter was converted completely to energy? Can a finite amount of energy be compressed into an infinitely small space. Nobody knows, and there are a lot of competing theories. We are not even sure what happens to space, itself, under those conditions.

Can space become infinitely large - as in the expanding universe expanding forever, for an infinite amount of time? Can it become infinitely small, such as postulated at the big bang? Would an observer inside the "infinitely small" universe or black hole core perceive it as small, or would it look like what we are seeing now? Our equations for physical processes become "undefined" at such points in our calculations, so we say "infinite" and "infinitesimal" and "singularity". Which roughly translate to "we don't understand that well enough to make quantitative statements about it."